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
- The most common modification is proteolytic cleavage. Some of the pre-cleavage polypeptides are immediately cleaved, while others are stored as inactive precursors to form a pool of enzymes (or other kinds of proteins) that can be activated very quickly, on a timescale of seconds to minutes, as compared to having to go through transcription and translation, or even just translation.
- 11.2: Protein Trafficking
- A major class of cleaved peptide sequences is signal peptides. Signal peptides direct the protein from the cytoplasm into a particular cellular compartment. In the case of prokaryotes, this essentially means the cell membrane, but for eukaryotes, there are specific signal peptides that can direct the protein to the nucleus, to the mitochondria, to the endoplasmic reticulum, and other intracellular organelles.
- 11.3: Protein Folding in the Endoplasmic Reticulum
- The ER lumen plays four major protein processing roles: folding/refolding of the polypeptide, glycosylation of the protein, assembly of multi-subunit proteins, and packaging of proteins into vesicles. Refolding of proteins is an important process because the initial folding patterns as the polypeptide is still being translated and unfinished may not be the optimal folding pattern once the entire protein is available. This is true for H-bonds and more permanent disulfide bonds as well.
- 11.4: N-linked Protein Glycosylation Begins in the ER
- Glycosylation is an important modification to eukaryotic proteins because the added sugar residues are often used as molecular flags or recognition signals to other cells than come in contact with them. There are two types of protein glycosylation, both of which require import of the target polypeptide into the ER. N-linked glycosylation actually begins in the endoplasmic reticulum, but O-linked glycosylation does not occur until the polypeptide has been transported into the Golgi apparatus.
- 11.5: O-linked Protein Glycosylation Takes Place Entirely in the Golgi
- O-linked glycoproteins begin their glycosylation with the action of the Golgi-specific enzyme, GalNAc transferase, which attaches an N-acetylgalactosamine to the hydroxyl group of a serine or threonine. Determining which residue to glycosylate appears to be directed by secondary and tertiary structure and often occurs in dense clusters of glycosylation. The combined oligosaccharide chains attached to an O-linked glycoprotein can contribute over 50% of the mass of a glycoprotein.
- 11.6: Vesicular Transport
- Vesicles (membrane-bound bubbles, essentially) pinch off from the ER, Golgi, and other membranous organelles, carrying with them whatever soluble molecules were inside the uid that was enclosed as well as any molecules embedded in that section of membrane. These vesicles then catch a ride on a molecular motor such as kinesin or myosin, and travel along the cytoskeleton until they dock at the appropriate destination and fuse with the target membrane or organelle.
Thumbnail: N-linked protein glycosylation (N-glycosylation of N-glycans) at Asn residues (Asn-x-Ser/Thr motifs) in glycoproteins. Image used with permission (Public Domain; Kosi Gramatikoff).