Principles of Protein Structural Organization - H. Schulz 1982

Covalent Structure of Proteins
Enzyme-Controlled Modifications of the Main Chain
Signal Sequences in Proteins

N-terminal chain extension apparently serves as a tagging mechanism. Many Newly synthesized Proteins are exported via The Cell's secretory machinery; others function intracellularly within the Cytosol, membranes, or their surrounding environment. The ultimate destination of a polypeptide may be encoded within its sequence, which is at least highly probable for exported proteins. According to the "signal hypothesis" [149], secretory proteins are synthesized as "proproteins"—precursors featuring a short-lived fragment attached to the N-terminus.

Elongated, so-called signal Peptides comprising roughly 15 predominantly hydrophobic amino acid residues have been identified in the precursors of certain immunoglobulin light chains [134, 150, 151], in proparathyroid hormone [152], and in various pancreatic proteins [153] (Fig. 4.5). As the signal peptide emerges from the ribosome, it is hypothesized to attach to the microsomal membrane, marking the initial step in a sequence of events that drives the translocation of the nascent chain across the membrane. During or immediately following this translocation, the signal sequence is cleaved by a membrane-bound protease.