Protein cotranslational processing

Acylation and prenylation. The amino terminus (usually glycine) of the a subunit of any G protein is nearly always converted to an N-myristoyl group.220-223 This modification occurs in a cotranslational process after removal of the initiating methionine (Chapter 29) and can be described as an acyl transfer from coenzyme A.224 The C termini of the y subunits of heterotrimeric G proteins, and also of the monomeric proteins of the Ras family, also undergo processing. For example, the C-terminal end of an intact Ras protein contains 18 residues which probably assume a largely a-helical conformation. A cysteine side chain near the terminus and having the sequence CAAX is... 

In . coli, the signal peptides of some envelope proteins are cleaved cotranslationally, while other precursors are completely synthesized before processing can be detected. While MBP displays both modes of processing, cotranslational processing does not occur until the precursor is 80% complete (Josefsson and Randall, 1981). Since the catalytic domain of signal peptidase is on the periplasmic face of the cytoplasmic membrane (Ohno-Iwashita and Wickner, 1983), cotranslational pro-... 

Arfin, S. M., and Bradshaw, R. A. (1988). Cotranslational processing and protein turnover in eukaryotic cells. Biochemistry 27, 7984-7990. 

Secretory proteins and proteins destined for membranes distal to the ER completely traverse the membrane bilayer and are discharged into the lumen of the ER. 7V-Glycan chains, if present, are added (Chapter 47) as these proteins traverse the irmer part of the ER membrane—a process called cotranslational glycosyla-tion. Subsequently, the proteins are found in the... 

N-Myristoylation is achieved by the covalent attachment of the 14-carbon saturated myristic acid (C14 0) to the N-terminal glycine residue of various proteins with formation of an irreversible amide bond (Table l). 10 This process is cotranslational and is catalyzed by a monomeric enzyme called jV-myri s toy 11ransferase. 24 Several proteins of diverse families, including tyrosine kinases of the Src family, the alanine-rich C kinase substrate (MARKS), the HIV Nef phosphoprotein, and the a-subunit of heterotrimeric G protein, carry a myr-istoylated N-terminal glycine residue which in some cases is in close proximity to a site that can be S-acylated with a fatty acid. Functional studies of these proteins have shown an important structural role for the myristoyl chain not only in terms of enhanced membrane affinity of the proteins, but also of stabilization of their three-dimensional structure in the cytosolic form. Once exposed, the myristoyl chain promotes membrane association of the protein. 5 The myristoyl moiety however, is not sufficiently hydrophobic to anchor the protein to the membrane permanently, 25,26 and in vivo this interaction is further modulated by a variety of switches that operate through covalent or noncovalent modifications of the protein. 4,5,27 In MARKS, for example, multiple phosphorylation of a positively charged domain moves the protein back to the cytosolic compartment due to the mutated electrostatic properties of the protein, a so-called myristoyl-electrostatic switch. 28 ... 

As the ribosome synthesizes a new peptide chain, the chain usually begins to fold, creating regions of secondary and even incomplete tertiary structure. Enzymes then act on these folded residues to modify them. As noted above, an important modification that occurs at this stage is the formation of disulfide bridges. Another is the cleavage of the peptide backbone at specific sites, which may be important for the transport of the protein across membranes in the cell. These modifications occur during the process of translation, and so they are described as cotranslational modifications. 

They may enter the cytosol and fold quickly into a compact form. This may require only a few seconds, whereas the translation process in the ribosome may take many seconds. The folding will therefore be cotranslational.525 Depending upon the N-terminal signal peptide the protein may later unfold and pass through a membrane pore or translocon into the endoplasmic reticulum (ER), a mitochondrion, chloro-plast, or peroxisome. Wherever it is, it will be crowded together with thousands of other proteins. It will interact with many of these, and evolution will have enabled some of these to become chaperones (discussed in Chapter 10).526... 

Thus, the temporal relationship between synthesis and secretion in E. coli remains somewhat unclear. Silhavy et al. (1983) and Rhoads et al. (1984) proposed that the coupling between the two processes is not as tight for prokaryotic secretion as it appears to be for eukaryotic secretion into the ER. The mode of secretion may be protein-specific. Some proteins [such as TEM j8-lactamase (Josefsson and Randall, 1981)], may be exported primarily posttranslationally, whereas some [such as PhoS (Pag s et al., 1984) and amp C /3-lactamase (Josefsson and Randall, 1981)] may be exported primarily cotranslationally in vivo. At least one protein that is secreted cotranslationally in vivo E. coli alkaline phosphatase) (Smith et al., 1977) can be translocated posttranslationally into E. coli membrane vesicles in vitro (Chen et al., 1985). The results of Ryan and Bassford (1985), described above, suggest that cotranslational export may be the major mode of secretion of wild-type proteins in normal (i.e., whole and not mutated) cells, while posttranslational secretion may be a backup system for use in case of damage to the secretory apparatus. 

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