11: Protein Modification and Trafficking
- Page ID
- 16162
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)Once a polypeptide has been translated and released from the ribosome, it may be ready for use, but often it must undergo post-translational processing to become fully functional. While many of these processes are carried out in both prokaryotes and eukaryotes, the presence of organelles provides the need as well as some of the mechanisms for eukaryote-specific modifications such as glycosylation and targeting.
- 11.1: Proteolytic Cleavage
- This page discusses the significance of proteolytic cleavage in protein activation, transforming inactive precursors like proproteins into active forms without the need for transcription or translation. It highlights insulin and collagen as key examples: insulin is derived from preproinsulin through several cleavage steps, while collagen begins as procollagen that assembles before propeptide removal.
- 11.2: Protein Trafficking
- This page highlights the essential role of propeptide sequences, especially signal peptides, in protein maturation and localization in both prokaryotes and eukaryotes. It details how proteins are directed to organelles, emphasizing the functions of importin proteins, nuclear pore complexes, and GTPases in nuclear transport.
- 11.3: Protein Folding in the Endoplasmic Reticulum
- This page discusses the critical functions of the endoplasmic reticulum (ER) lumen in protein processing, including folding, glycosylation, and packaging. It highlights the role of protein disulfide isomerase (PDI) in rearranging disulfide bonds and the assistance of chaperone proteins in preventing premature folding. Additionally, it mentions chaperonins like the GroEL/GroES complex in bacteria, which aid in protein refolding by undergoing conformational changes.
- 11.4: N-linked Protein Glycosylation Begins in the ER
- This page discusses glycosylation, the addition of sugar residues to eukaryotic proteins, which aids in cellular recognition. It details two types: N-linked, initiated in the ER during translation, and O-linked, occurring in the Golgi post-translation. N-linked glycosylation involves asparagine and assists in protein folding, supported by calnexin and calreticulin.
- 11.5: O-linked Protein Glycosylation Takes Place Entirely in the Golgi
- This page explains the glycosylation of O-linked glycoproteins facilitated by GalNAc transferase, which attaches N-acetylgalactosamine to serine/threonine residues. The protein's structure influences this process, impacting interactions with other cells. Oligosaccharide chains increase glycoprotein mass and provide a hydrophilic coating that can mask the protein core, affecting properties such as NCAM's adhesion.
- 11.6: Vesicular Transport
- This page explains the secretion of concentrated proteins via the endoplasmic reticulum and Golgi apparatus, focusing on vesicle formation and transport mechanisms involving coat proteins like COPII, COPI, and clathrin. It covers vesicle targeting and fusion, emphasizing the importance of pH changes and matching v-SNARE and t-SNARE proteins for successful docking. Tethering proteins assist in vesicle approach to membranes.
- 11.7: Receptor-mediated Endocytosis
- This page explains endocytosis and lysosomal function, detailing how ligand binding initiates vesicle formation for material internalization, with a focus on cholesterol uptake via LDL. It emphasizes the endosomal pathway, lysosomal digestion through acid hydrolases, and the role of proton pumps, including lysosomal storage diseases.
Thumbnail: N-linked protein glycosylation (N-glycosylation of N-glycans) at Asn residues (Asn-x-Ser/Thr motifs) in glycoproteins. (Public Domain; Kosi Gramatikoff).