Learning Goals Associated with 2020_SS1_Bis2a_Facciotti_Reading_11
Living things fall into three large groups: Archaea, Bacteria, and Eukarya. The first two groups include non-nucleated cells, and the third contains all eukaryotes. A relatively sparse fossil record is available to help us discern what the first members of each of these lineages looked like, so it is possible that all the events that led up to the last common ancestor of extant eukaryotes will remain unknown. However, comparative biology of extant organisms and the limited fossil record provide some insights into the history of Eukarya.
The earliest fossils found appear to be bacteria, most likely cyanobacteria. They are about 3.5 billion years old and are recognizable because of their relatively complex structure and, for bacteria, relatively large cells. Most other bacteria and archaea have small cells, 1 or 2
Characteristics of eukaryotes
Data from these fossils have led biologists to the conclusion that living eukaryotes are all descendants of a single common ancestor. Mapping the characteristics found in all major groups of eukaryotes reveals that the following characteristics must have been present in the last common ancestor, because these characteristics are present in at least
- Cells with nuclei surrounded by a nuclear envelope with nuclear pores. This is the single characteristic that is both necessary and sufficient to define an organism as a eukaryote. All extant eukaryotes have cells with nuclei.
- Mitochondria. Some extant eukaryotes have very reduced remnants of mitochondria in their cells, whereas other members of their lineages have “typical” mitochondria.
- A cytoskeleton containing the structural and motility components called
actinmicrofilaments and microtubules. All extant eukaryotes have these cytoskeletal elements.
- Flagella and cilia, organelles associated with cell motility. Some extant eukaryotes lack flagella and/or cilia, but
they are descendedfrom ancestors that possessed them.
- Chromosomes, each
consisting ofa linear DNA molecule coiled around basic (alkaline) proteins called histones. The few eukaryotes with chromosomes lacking histones clearlyevolved from ancestors that had them.
- Mitosis, a process of nuclear division wherein replicated chromosomes
are dividedand separated using elements of the cytoskeleton. Mitosis is universally present in eukaryotes.
- Sex, a process of genetic recombination unique to eukaryotes in which diploid nuclei at one stage of the life cycle undergo meiosis to yield haploid nuclei and subsequent karyogamy, a stage where two haploid nuclei fuse
togetherto create a diploid zygote nucleus.
- Members of all major lineages have cell walls, and it might be reasonable to conclude that the last common ancestor could make cell walls during some stage of its life cycle. However, not enough
is knownabout eukaryotes’ cell walls and their development to know how much homology exists among them. If the last common ancestor could make cell walls, it is clear that this ability must have been lostin many groups.
Endosymbiosis and the evolution of eukaryotes
Bacterial and archaeal metabolism
Many important metabolic processes arose in bacteria and archaea, and some of these, such as nitrogen fixation,
While today’s atmosphere is about one-fifth molecular oxygen (O2), geological evidence shows that it originally lacked O2. Without oxygen, aerobic respiration would not
Eventually, the amount of photosynthetic oxygen built up in some environments to levels that posed a risk to living organisms, since it can damage many organic compounds. Various metabolic processes evolved that protected organisms from oxygen; one of which, aerobic respiration, also generated high levels of ATP. It became widely present among microbes, including in a group we now call alpha-
Recall that the first fossils that we believe to be eukaryotes are about 2 billion years old, so they appeared as oxygen levels were increasing. Also, recall that all extant eukaryotes descended from an ancestor with mitochondria. These organelles were first observed by light microscopists in the late 1800s, where they appeared to be worm-shaped structures that seemed to move around in the cell. Some early observers suggested that they might be bacteria living inside host cells, but these hypotheses remained unknown or rejected in most scientific communities.
As cell biology developed in the twentieth century,
Broadly, it has become clear that many of our nuclear genes and the molecular machinery responsible for replication and expression appear closely related to those in Archaea.
One of the major features distinguishing bacteria and archaea from eukaryotes is mitochondria. Eukaryotic cells may contain anywhere from one to several thousand mitochondria, depending on the cell’s level of energy consumption. Each mitochondrion measures 1 to 10 or greater micrometers
Eukaryotic Cell: Structure and Function
Introduction to eukaryotic cell structure
By definition, eukaryotic cells are cells that contain a membrane-bound nucleus, which is not present in bacterial or archaeal cells. Besides the nucleus, eukaryotic cells
In previous sections, we considered the Design Challenge of making cells larger than a small bacterium—more precisely, growing cells to sizes at which, in the eyes of natural selection, relying on diffusion of substances for transport through a highly viscous cytosol comes with inherent functional trade-offs that offset most selective benefits of getting larger. In the lectures and readings on bacterial cell structure, we discovered some morphological features of large bacteria that allow them
As we transition our focus to eukaryotic cells, we want you to approach the study by constantly returning to the Design Challenge. We will cover many subcellular structures unique to eukaryotes, and you will
Figure 1. These figures show the major organelles and other cell components of (a) a typical animal cell and (
The plasma membrane
Like bacteria and archaea, eukaryotic cells have a plasma membrane, a phospholipid bilayer with embedded proteins that separates the internal contents of the cell from its surrounding environment. The plasma membrane controls the passage of organic molecules, ions, water, and oxygen into and out of the cell. Wastes (such as carbon dioxide and ammonia) also leave the cell by passing through the plasma membrane, usually with some help of protein transporters.
Figure 2. The eukaryotic plasma membrane is a phospholipid bilayer with proteins and cholesterol embedded in it.
As discussed in bacterial cell membranes, the plasma membranes of eukaryotic cells may also adopt unique structural conformations. For instance, in cells that specialize in absorption the plasma membrane often fold into fingerlike projections called microvilli (singular = microvillus) (see figure below). The "folding" of the membrane into microvilli increases the surface area for absorption while minimally impacting the cytosolic volume. We can find such cells lining the small intestine, the organ that absorbs nutrients from digested food.
An aside: People with celiac disease have an immune response to gluten, a protein found in wheat, barley, and rye. The immune response damages microvilli. As a result, afflicted individuals have an impaired ability to absorb nutrients. This can lead to malnutrition, cramping, and diarrhea.
Figure 3. Microvilli, shown here as they appear on cells lining the small intestine, increase the surface area available for absorption.
The cytoplasm refers to the entire region of a cell between the plasma membrane and the nuclear envelope.
Typically, the nucleus is the most prominent organelle in a cell (see figure below) when viewed through a microscope. The nucleus (plural = nuclei) houses the cell’s DNA. Let’s look at it in more detail.
Figure 4. The nucleus stores chromatin (DNA plus proteins) in a gel-like substance called the nucleoplasm. The nucleolus is a condensed region of chromatin where ribosome synthesis occurs. The boundary of the nucleus
The nuclear envelope
The nuclear envelope is a double phospholipid bilayer that
Chromatin and chromosomes
Chromosomes are structures within the nucleus that
Chromosomes are only
Figure 5. (a) This image shows various levels of the organization of chromatin (DNA and protein). (b) This image shows paired chromosomes. Credit (
Some chromosomes have sections of DNA that encode ribosomal RNA. A darkly staining area within the nucleus called the nucleolus (plural = nucleoli) aggregates the ribosomal RNA with associated proteins to assemble the ribosomal subunits that
Ribosomes are the cellular structures responsible for the process of protein synthesis referred to as translation. When viewed through an electron microscope, ribosomes appear either as clusters (polyribosomes) or single, tiny dots that float freely in the cytoplasm. They may also appear to
Electron microscopy has shown us that ribosomes, which are large complexes of protein and RNA,
Mitochondria (singular = mitochondrion)
The structure of the mitochondria can vary significantly depending on the organism and the state of the cell cycle which one is observing. The typical textbook image, however, depicts mitochondria as oval-shaped organelles with a double inner and outer membrane (see figure below); learn to recognize this generic representation. Both the inner and outer membranes are phospholipid bilayers embedded with proteins that mediate transport across them and catalyze various other biochemical reactions. The inner membrane layer has folds called cristae that increase the surface area into which
Figure 7. This electron micrograph shows a mitochondrion as viewed with a transmission electron microscope. This organelle has an outer membrane and an inner membrane. The inner membrane contains folds, called cristae, which increase its surface area. The space between the two membranes
There are many other organelles, but they all serve essential functions to the cell. Let’s introduce a few more.
Peroxisomes are small, round organelles enclosed by single membranes. These organelles carry out chemical reactions known as redox reactions that oxidize and break down fatty acids and amino acids. They also help to detoxify many toxins that may enter the body. Many of these redox reactions release hydrogen peroxide, H2O2, which would be damaging to cells; however, when these reactions are confined to peroxisomes, enzymes safely break down the H2O2 into harmless oxygen and water. For example, alcohol is detoxified by peroxisomes in liver cells. Glyoxysomes, which are specialized peroxisomes in plants, are responsible for converting stored fats into sugars.
Animal cells have another set of organelles not found in plant cells: lysosomes. Colloquially, the lysosomes are sometimes called the cell’s “garbage disposal”. Enzymes within the lysosomes aid the breakdown of proteins, polysaccharides, lipids, nucleic acids, and even "worn-out" organelles. These enzymes are active at a much lower pH than that of the cytoplasm. Therefore, the pH within lysosomes is more acidic than the pH of the cytoplasm. In plant cells, many of the same digestive processes take place in vacuoles.
Vesicles and vacuoles
Vesicles and vacuoles are membrane-bound sacs that function in storage and transport. Other than the fact that vacuoles are somewhat larger than vesicles, there is a very subtle distinction between them: the membranes of vesicles can fuse with either the plasma membrane or other membrane systems within the cell. Additionally, some agents such as enzymes within plant vacuoles break down macromolecules. The membrane of a vacuole does not fuse with the membranes of other cellular components.
The central vacuole
Vacuoles as essential components of plant cells. If you look at the cartoon figure of the plant cell, you will see that it depicts a large central vacuole that occupies most of the area of the cell. The central vacuole plays a key role in regulating the cell’s concentration of water in changing environmental conditions.
The central vacuole also supports the expansion of the cell. When the central vacuole holds more water, the cell gets larger without having to invest a lot of energy in synthesizing new cytoplasm.
The centrosome is an organelle that serves as one of the main sites from which microtubules originate in animal and yeast cells. It is also a microtubule-organizing center found near the nuclei of animal cells. It contains a pair of centrioles, two structures that lie perpendicular to each other (see figure below). Each centriole is a cylinder of nine triplets of microtubules.
Figure 8. The centrosome consists of two centrioles that lie at right angles to each other. Each centriole is a cylinder made up of nine triplets of microtubules. Nontubulin proteins (indicated by the green lines) hold the microtubule triplets together.
The centrosome replicates itself before a cell divides, and the centrioles appear to have some role in pulling the duplicated chromosomes to opposite ends of the dividing cell.
The cell wall
If you examine the diagram above depicting plant and animal cells, you will see in the diagram of a plant cell a structure external to the plasma membrane called the cell wall. The cell wall is a rigid covering that protects the cell, provides structural support, and gives shape to the cell.
Fungal and protistan cells also have cell walls. While the chief component of bacterial cell walls is peptidoglycan, the major organic molecule in the plant cell wall is cellulose (see structure below), a polysaccharide made up of glucose subunits.
Figure 9. Cellulose is a long chain of β-glucose molecules connected by a 1-4 linkage. The dashed lines at each end of the figure indicate a series of many more glucose units. The size of the page makes it impossible to portray an entire cellulose molecule.
Chloroplasts are plant cell organelles that carry out photosynthesis. Like the mitochondria, chloroplasts have their own DNA and ribosomes, but chloroplasts have an entirely different function.
Like mitochondria, chloroplasts have outer and inner membranes, but within the space enclosed by a chloroplast’s inner membrane is a set of interconnected and stacked fluid-filled membrane sacs called thylakoids (figure below). Each stack of thylakoids is called a granum (plural = grana). The fluid enclosed by the inner membrane that surrounds the grana is called the stroma.
Figure 10. The chloroplast has an outer membrane, an inner membrane, and membrane structures called thylakoids that are stacked into grana. The space inside the thylakoid membranes is called the thylakoid space. The light harvesting reactions take place in the thylakoid membranes, and the synthesis of sugar takes place in the fluid inside the inner membrane, which is called the stroma. Chloroplasts also have their own genome, which is contained on a single circular chromosome.
The chloroplasts contain a green pigment called chlorophyll, which captures the light energy that drives the reactions of photosynthesis. Like plant cells, photosynthetic protists also have chloroplasts. Some bacteria perform photosynthesis, but their chlorophyll is not relegated to an organelle.
EVOLUTION CONNECTION: ENDOSYMBIOSIS
We have mentioned that both mitochondria and chloroplasts contain DNA and ribosomes. Have you wondered why? Strong evidence points to endosymbiosis as the explanation.
Symbiosis is a relationship in which organisms from two separate species depend on each other for their survival. Endosymbiosis (endo- = “within”) is a mutually beneficial relationship in which one organism lives inside the other. Endosymbiotic relationships abound in nature. For instance, some microbes that live in our digestive tracks produce vitamin K. The relationship between these microbes and us (their hosts) is said to be mutually beneficial or symbiotic. The relationship is beneficial for us because we are unable to synthesize vitamin K; the microbes do it for us instead. The relationship is also beneficial for the microbes because they receive abundant food from the environment of the large intestine, and they are protected both from other organisms and from drying out.
Scientists have long noticed that bacteria, mitochondria, and chloroplasts are similar in size. We also know that bacteria have DNA and ribosomes, just as mitochondria and chloroplasts do. Scientists believe that host cells and bacteria formed an endosymbiotic relationship when the host cells ingested both aerobic and autotrophic bacteria (cyanobacteria) but did not destroy them. Through many millions of years of evolution, these ingested bacteria became more specialized in their functions, with the aerobic bacteria becoming mitochondria and the autotrophic bacteria becoming chloroplasts. There will be more on this later in the reading.
The Endoplasmic Reticulum
The endoplasmic reticulum (ER) (see figure above) is a series of interconnected membranous sacs and tubules that collectively
The hollow portion of the ER tubules
The rough endoplasmic reticulum (RER) is so named because the ribosomes attached to its cytoplasmic surface give it a studded appearance when viewed through an electron microscope (see figure below).
This transmission electron micrograph shows the rough endoplasmic reticulum and other organelles in a pancreatic cell. (credit: modification of work by
Ribosomes transfer their newly synthesized proteins into the lumen of the RER where they undergo structural modifications, such as folding or the acquisition of side chains. These
If the phospholipids or
Since the RER is engaged in
The smooth endoplasmic reticulum (SER) is continuous with the RER but has few or no ribosomes on its cytoplasmic surface. Functions of the SER include
In muscle cells, a specialized SER called the sarcoplasmic reticulum is responsible for storage of the calcium ions that
The Golgi Apparatus
We have already mentioned that vesicles can bud from the ER and transport their contents elsewhere, but where do the vesicles go? Before reaching their final destination, the lipids or proteins within the transport vesicles still need to
The Golgi apparatus in this white blood cell is visible as a stack of semicircular, flattened rings in the lower portion of the image.
The receiving side of the Golgi apparatus
In another example of form following function, cells that engage in a great deal of secretory activity (such as cells of the salivary glands that secrete digestive enzymes or cells of the immune system that secrete antibodies) have an abundance of Golgi.
In plant cells, the Golgi apparatus has the additional role of synthesizing polysaccharides, some of which
A macrophage has engulfed (phagocytized) a potentially pathogenic bacterium and then fuses with
Possible NB Discussion Point
If you were given a lab and the authority to pursue research in one of the organelles you just learned about, which one would you want to conduct research on? Why? What specific aspect of the organelle would you want to learn more about?
The cytoskeleton is a network of different protein fibers that provides many functions: it maintains or changes the shape of the cell; it secures some organelles in specific positions; it enables movement of cytoplasm and vesicles within the cell; and it enables the cell to move in response to stimuli. There are three types of fibers within the cytoskeleton: microfilaments, intermediate filaments, and microtubules. Some of the cytoskeletal fibers work in conjunction with molecular motors which move along the fibers within the cell to carry out a diverse set of functions. There are two main families of cytoskeletally-associated molecular motors: dyneines and kinesins.
Figure 1. Microfilaments thicken the cortex around the inner edge of a cell; like rubber bands, they resist tension. Microtubules are found in the interior of the cell where they maintain cell shape by resisting compressive forces. Intermediate filaments are found throughout the cell and hold organelles in place.
Problem statement: Eukaryotic cells contain membrane-bound organelles that effectively separate materials, processes, and reactions from one another and from the cytoplasm. This in itself poses a problem for eukaryotes.
How can the cell purposely move and control the location of materials between these organelles? More specifically, how can a eukaryotic cell transport compounds from their place of origin (in most cases the cytoplasm) to where they are needed (perhaps the nucleus, the mitochondria, or the cell surface)?
One possible solution is for the cell to create a network that can connect all the different parts of the cell together. This network could be used not only as a scaffold to hold components in place but also as a reference for direction. For example, we can use a map to determine the direction we need to travel and roads to connect and travel from home to campus. Likewise, an interconnecting network inside the cell can be used to direct and move compounds from one location to a final destination. Some of the required characteristics of this network are listed below. Can you add to this list?
- The network needs to be extensive, and connect every area of the cell.
- The network needs to be flexible, able to change and adapt as the cell grows larger, divides into two cells, or physically moves from one environment to another.
- The network needs to be strong, able to hold up to mechanical pressure from inside the cell or from outside of the cell.
- The network needs to be composed of different fibers and each of these fibers needs to be for a specific connection in the cell. For example, certain fibers might be involved in holding organelles in place, and other fibers would be involved in connecting two different organelles.
- The fibers need to have directionality (or polarity), meaning they need to have a defined starting point and a defined end to help direct movement from one location to another.
- The fibers need to work with proteins that can convert chemical energy into kinetic energy, to actively transport compounds along the fibers.
Microfilaments are cytoskeleton fibers composed of actin subunits. Actin is one of the most abundant proteins in eukaryotic cells and comprises 20% of total cellular protein by weight in muscle cells. The actin amino acid sequence is highly conserved in eukaryotic cells, meaning that the protein amino acid sequence, and therefore its final 3-D shape, has changed little over the course of evolution, maintaining more than 80% similarity between algae and humans.
Actin can be present as either a free monomer called G-actin (globular) or as part of a polymer microfilament called F-actin ("F" for filamentous). Actin must be bound to ATP in order to assemble into its filamentous form and maintain the structural integrity of the filament. The actin filament itself has structural polarity. This term "polarity", in reference to a cytoskeleton filament, does not mean what it did when we discussed polar functional groups earlier in this course. Polarity here refers to the fact that there are two distinct ends to the filament. These ends are called the "(-)" end and the "(+)" end. At the "(+)" end, actin subunits are adding onto the elongating filament and at the "(-)" end, actin subunits are disassembling or falling off of the filament. This process of assembly and disassembly is controlled by the ATP to ADP ratio in the cytoplasm.
Figure 2. Microfilaments are the narrowest of the three cytoskeleton fibers, with a diameter of about seven nm. Microfilaments are composed of actin subunits which form into two intertwined strands.
Actin participates in many cellular processes, including muscle contraction, cell motility, cytokinesis during cell division, vesicle and organelle movement, and the maintenance of cell shape. Actin filaments serve as a track for the movement of a family of motor proteins called myosins discussed in more detail in a section below.
Link to learning:
To see an example of a white blood cell in action, click here and watch a short time-lapse video of the cell capturing two bacteria. It engulfs one and then moves on to the other.
Animations on actin filaments and how they work
Intermediate filaments are made of several strands of fibrous proteins that are wound together. These elements of the cytoskeleton get their name from the fact that their diameter, eight to ten nm, is between those of the smaller microfilaments and the larger microtubules. The intermediate filaments are the most diverse group of cytoskeletal elements. Several types of fibrous proteins are found in the intermediate filaments. You are probably most familiar with keratin, the fibrous protein that strengthens your hair, nails, and the epidermis of the skin.
Figure 3. Intermediate filaments consist of several intertwined strands of fibrous proteins.
Intermediate filaments have no role in cell movement. Their function is purely structural. They bear tension, thus maintaining the shape of the cell, and anchor the nucleus and other organelles in place. The figure above shows how intermediate filaments create a cable-like supportive scaffolding inside the cell.
Microtubules are the largest component of the cytoskeleton and are found throughout the cytoplasm. These polymers are made up of globular protein subunits called α-tubulin and β-tubulin. Microtubules are found not only in eukaryotic cells but in some bacteria as well.
Both the α-tubulin and β-tubulin subunits bind to GTP. When bound to GTP, the formation of the microtubule can begin, this is called the nucleation event. As more GTP tubulin dimers assemble onto the filament, GTP is slowly hydrolyzed by β-tubulin to form GDP. Tubulin bound to GDP is less structurally robust and can lead to disassembly of the microtubule.
Much like the actin filaments discussed above, microtubules also have a distinct polarity that is critical for their biological function. Tubulin polymerizes end to end, with the β-subunits of one tubulin dimer contacting the α-subunits of the next dimer. These differences lead to different subunits being exposed on the two ends of the filament. The ends are designated the "(−)" and "(+)" ends. Unlike actin filaments, microtubules can elongate at both the "(+)" and "(-)" ends, but elongation is significantly more rapid at the "(+)" end.
Figure 4. Microtubules are hollow. Their walls consist of 13 polymerized dimers of α-tubulin and β-tubulin (right image). The left image shows the molecular structure of the tube.
Microtubules help the cell resist compression, provide a track along which vesicles move through the cell, pull replicated chromosomes to opposite ends of a dividing cell, and are the structural elements of flagella, cilia, and centrioles (the latter are the two perpendicular bodies of the centrosome). In fact, in animal cells, the centrosome is the microtubule organizing center. In eukaryotic cells, flagella and cilia are quite different structurally from their counterparts in bacteria, discussed below.
Where did these fibers come from?
The cytoskeleton probably has its origins in bacterial and/or archaeal ancestry. There are ancient relatives to both actin and tubulin in bacterial systems. In bacteria, the MreB protein and the ParM protein are believed to be early ancestors to Actin. MreB functions in maintaining cell shape and ParM functions in plasmid (DNA) partitioning. The FtsZ protein in bacteria functions in cytokinesis, it is a GTPase, spontaneously forms filaments and is hypothesized to be an ancient form of tubulin. These findings support the hypothesis that the eukaryotic cytoskeleton has its origins in the bacterial world.
Flagella and cilia
Flagella (singular=flagellum) are long, hair-like structures that extend from the plasma membrane and are used to move an entire cell (for example, sperm, Euglena). When present, the cell has just one flagellum or a few flagella. Cilia are short, hair-like structures that are used to move entire cells (such as paramecia) or substances along the outer surface of the cell (for example, the cilia of cells lining the fallopian tubes that move the ovum toward the uterus, or cilia lining the cells of the respiratory tract that trap particulate matter and move it toward your nostrils.) When cilia are present, there can be many of them, extending along the entire surface of the plasma membrane.
Despite their differences in length and number, flagella and cilia share a common structural arrangement of microtubules called a “9+2 array.” This is an appropriate name because a single flagellum or cilium is made of a ring of nine microtubule doublets, surrounding a single microtubule doublet in the center (Figure 5).
Figure 5. This transmission electron micrograph of two flagella shows the "9+2 array" of microtubules: nine microtubule doublets surround a single microtubule doublet. (credit: modification of work by Dartmouth Electron Microscope Facility, Dartmouth College; scale-bar data from Matt Russell)
For a video on flagellar and ciliar movement in eukaryotes, see the YouTube video: click here (you can skip the commericial).
One function of the cytoskeleton is to move cellular components from one part of the cell to another. These cellular components are called "cargo" and are often stored within a vesicle for transport. You can think of the cytoskeleton as "railroad tracks" providing support and directionality inside of the cell.
Of course, if there are "railroad tracks" there needs to be an engine that can both move on the tracks and pull or push cargo along. In this case the engines are molecular motors that can move along the tracks in a specific direction. There are two families of molecular motors associated with the cytoskeleton; dyneines and kinesins. These motor proteins (train engines) and the cytoskeleton create a comprehensive network within the cell for moving vesicles (box cars) from one organelle to another or from one organelle to the cell surface.
Figure 6. Organelle transport via microtubules and kinesins and dynes. Note that the figure is conceptual and only intended to show directionality of movement of various organelles; it does not necessarily represent all of their forms faithfully.
Dynein is a protein complex that functions as a molecular motor. In cells, it converts the chemical energy from ATP hydrolysis into the mechanical energy of movement to 'walk' along the microtubule while carrying a vesicle. Dyneins bind to microtubules and move or "walk" from the plus "(+)" end of the cytoskeletal microtubule filament to the minus "(-)" end of the filament, which is usually oriented towards the cell center. Thus, they are often referred to as "minus end directed motors" and this vesicular transport is refereed to as retrograde transport. Cytoplasmic dynein moves processively along the microtubule, hydrolyzing ATP with each "step" it takes along the microtubule. During this process, one or the other of its "stalks" is always attached to the microtubule, allowing for the dynein motor (and its cargo) to "walk" a considerable distance along a microtubule without detaching.
Figure 7. Schematic of cytoplasmic dynein motor protein. Dyneins are protein complexes composed of many smaller polypeptide subunits. The overall structure of the dynien motors are relatively simple, consisting of two identical complexes each having a motor domain that interacts with the microtubule, a stalk, or stem region that connects the motor head to the cargo interacting domain.
Cytoplamic dyneins are used in many different processes: they are involved in organelle movement such as the positioning of the Golgi complex and other organelles in the cell; they are used in the transport of cargo such as the movement of vesicles made by the endoplasmic reticulum, endosomes, and lysosomes; and they are responsible for the movement of chromosomes during cell division. Axonemal dyneins are motor proteins used in the sliding of microtubules in the axonemes of cilia and flagella in eukaryotic cells.
Kinesins, like cytoplasmic dyneins are motor-protein complexes that "walk" along the microtubules and are involved in vesicle transport. Unlike cytoplasmic dyneins, the polarity of kinesin movement is from the "(-)" end of the microtubule to the "(+)" end with the hydrolysis of ATP. In most cells, this entails transporting cargo from the center of the cell towards the periphery (the opposite direction to dyneins). This form of transport is known as anterograde or orthrograde transport. Like cytoplasmic dyneins, kinesins are involved in a variety of cellular processes including vesicle movement and chromosome movement during cell division.
The structure of kinesins are similar to cytoplasmic dyneins and is diagrammed in Figure 8. Members of the kinesin superfamily vary in shape, but the overall structure is that of a heterotetramer whose motor subunits (heavy chains) form a protein dimer (molecule pair) that binds two light chains.
Figure 8. Schematic of kinesin motor proteins. The heavy chains comprise a globular head (the motor domain) at the amino terminal end connected via a short, flexible neck linker to the stalk—a long, central α-helical coiled-coil domain—that ends in a carboxy terminal tail domain which associates with the light-chains. The stalks of two light chains intertwine to form a coiled-coil that directs dimerization of the two heavy chains. In most cases transported cargo binds to the kinesin light chains, but in some cases cargo binds to the C-terminal domains of the heavy chains.
Possible NB Discussion Point
Explain the functions and importance of dyneins and kinesins as if you were trying to explain this to your 10 year old cousin. How are they similar and how are they different?
Animations of kinesin and dynein at work
How do the motors interact with cargo and the microtubules?
Cytoplasmic dyneins and kinesins interact with both cargo and microtubules in similar fashion. The light chains interact with receptors on the various cargo vesicles and the globular motor domains, specifically interact with the microtubules.
Figure 9. Schematic of kinesin motor protein carrying a cargo vesicle along a microtubule filament.