Microsequencing of Proteins of the Rough Endoplasmic Reticulum (rER) Membrane
The ER membrane is an important organelle involved in such diverse functions as protein translocation, protein folding and phospholipid biosynthesis. It is the entry point for most membrane and soluble proteins into the secretory pathway. Most proteins destined for translocation into the ER contain a signal sequence at their N-terminus consisting of basic amino acids followed by a stretch of hydrophobic amino acids. The SRP (signal-recognition particle) binds to the signal sequence of a nascent secretory protein that is bound to a ribosome. The ribosome-SRP complex is then bound to its receptor (DP, docking protein) in the ER membrane. After release of SRP, the nascent proteins are inserted into the membrane or trans located into the lumen of the ER (Walter and Lingappa, 1986; Rapoport, 1992) by means of a number of membrane proteins like TRAMp and the Sec6l p complex. In the lumen of the ER, the synthesized polypeptide may undergo ER-specific cotranslational and post-translational modifications such as cleavage of the signal peptide, disulfide bond formation, N-linked glycosylation, fatty acylation, or prolyl hydroxylation. Thus, a multitude of functions are carried out by ER proteins which either integrate into the membrane or are located in the lumen. Up to now none of these functions is completely understood because not all proteins involved in these processes have been identified.
KeywordsProtein Disulfide Isomerase Protein Translocation Spot Number Phospholipid Biosynthesis FASTA Computer Program
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