Add time:08/08/2019         Source:sciencedirect.com

Publisher SummaryThis chapter discusses the application of the signal hypothesis to the incorporation of integral membrane proteins. Signal hypothesis is a proposed explanation of the biosynthesis of secreted polypeptides. It has become increasingly clear that much of the knowledge may be extended to membrane proteins inserted into the rough endoplasmic reticulum (RER) membrane. There exists strong evidence that the basic process of translocation across the membrane is the same for both types of polypeptides: the signals for initiation of the process are the same, and the constituents of the transport machinery are identical. Most of the details on the mechanism of translocation are studied on secretory proteins. Translocation across the endoplasmic reticulum membrane and the cytoplasmic membrane in bacteria has turned out to be very similar. According to the signal hypothesis, the polypeptide chain would cross the hydrophobic phospholipid bilayer through a protein channel. During or shortly after completion of the protein, an enzyme, called “signal peptidase,” which is located at the luminal side of the endoplasmic reticulum, cleaves off the signal peptide. When the ribosome has reached the stop codon on the mRNA, the translation complex disintegrates, the pore in the membrane disappears, and the ribosome becomes free again.

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