16.5: A Diversity of Membrane Protein Functions
Example Membrane Protein Functions
- receptors for hormones or neurotransmitters
- antibodies of the immune system that recognize foreign substances (antigens)
- cell-recognition molecules that bind cells together
- cell membrane structures that directly pass chemical information between cells
- anchoring cells to extracellular surfaces like connective tissue
- molecular transport (entry into or exit of substances from cells)
- enzymes that catalyze crucial reactions in cells.
Transmembrane proteins perform most of the functions illustrated here. However, peripheral membrane proteins also play vital roles in membrane function. Remember that Cytochrome c in the electron transport system on the mitochondrial cristal membrane is a peripheral protein. Other peripheral membrane proteins may serve to regulate the transport or signaling activities of transmembrane protein complexes or may mediate connections between the membrane and cytoskeletal elements. The peripheral membrane proteins by shown here do not penetrate membranes. They bind reversibly to the internal or external surfaces of the biological membrane with which they are associated. We will be looking more closely at what holds membrane proteins in place and how they perform their unique functions. Check out major membrane protein functions, actions and cellular locations below
| Basic Function | Specific Actions | Examples |
| Facilitated Transport | Regulate diffusion of substances across membranes along a concentration gradient | Ca 2 + & other ion channels, glucose transporters |
| Active Transport | Use energy to move ions from low to high concentrations across membranes | Mitochondrial protein pumps, the Na + /K + ion pump in neurons |
| Signal Transduction | For e.g., hormones that can't enter cells, these convey information from molecular signals to cytoplasm, leading to a cellular response | Protein hormone and growth factor signaling, antibody/antigen interactions, cytokine mediation of inflammatory responses etc. |
| Cell-cell interactions | Cell-cell recognition and binding to form tissues | Formation of desmosomes gap junctions and tight junctions. |
| Anchors to cytoskeleton | Link membrane proteins to cytoskeleton | Give cells their shape, cell movement and response to molecular signals |
| Enzymatic | Usually multifunctional proteins with enzymatic activities | The F1 ATP synthase that uses proton gradient to make ATP; adenylyl cyclase that makes cAMP during signal transduction; note that some receptor proteins are linked to enzymatic domains in the cytoplasm |
288 Diversity of Membrane Protein Structure and Function