9 Secretory and membrane protein biosynthesis
Biosynthesis
Like cytosolic proteins, membrane proteins and those to be secreted or targeted to lysosomes are synthesized against the mRNA template by ribosomes. However, before synthesis progresses very far, the translation of these proteins is halted until the ribosome has been transferred to the rough endoplasmic reticulum (ER). A characteristic hydrophobic amino acid sequence of 18–30 residues flanked by basic residues at the N-terminus of the nascent polypeptide, termed the signal or leader sequence, is recognized by a large protein–RNA complex called the signal recognition particle (SRP) (Fig. 3.9.1). Binding of the SRP to the polypeptide–ribosome complex prevents further protein synthesis while the ribosome is in the cytoplasm.
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