Processing Mechanisms in the Biosynthesis of Proteins*

Abstract

Limited proteolysis is a widely occurring mechanism in protein biosynthesis. Protein precursors can be classified according to their functions, localization within cell compartments, and enzymic cleavage mechanisms. The presecretory proteins represent an important class of very rapidly turning over precursors which play an early role in the sequestration of secretory products and whose cleavage appears to be intimately associated with structures formed at the ribosome-membrane junction during protein synthesis. A model is proposed which predicts that the prepeptide forms a beta-pleated sheet structure with other components of the membrane which results in the transfer of a loop of peptide across the microsomal membrane. Proinsulin is representative of the general class of proproteins that are processed post-translationally within their secretory cells either during the formation and maturation of secretory granules (peptides hormones and neurotransmitters, serum albumins) or during the assembly of macromolecular structures (virus capsules, membrane-associated enzyme complexes). The former group are cleaved by Golgi-associated proteases having tryptic and carboxypeptidase B-like specificity. Some precursors are secreted as such and processed extracellularly either in the circulation or at special sites (procollagens, zymogens, provenoms, vitellogenins).

Nucleotide sequence analysis of cDNAs encoding a bovine galanin precursor protein in the adrenal medulla and chemical isolation of bovine gut galanin

“…The translation initiation site is tentatively assigned to the methionine codon ATG at nucleotide positions l-3, which is the first initiating triplet in the reading frame upstream from galanin ( fig.2). The size and hydrophobicity of the peptide following this tentatively assigned initiating methionine is 29 amino acids, in agreement with the size usually found in leader peptides [19,20] and identical in length to that found in the pig [2] but different at three amino acid positions ( fig.2). Two of these three amino acid substitutions are changes between hydrophobic amino acids.…”

Cloning and sequence analysis of cDNA for rat corticotropin‐releasing factor precursor

“…The translational initiation site assigned is corroborated by the finding that this site represents the first AUG triplet that appears downstream of a nonsense codon (UGA at positions -144 to -142) found in frame. The sequence of the 24 amino acid residues starting with the initiating methionine exhibits a feature characteristic of the signal peptide of secretory proteins [12,13]. A possible site for cleavage of the signal peptide seems to be located after the alanine residue specified by the 24th codon (alternatively after the serine residue specified by the 27th codon).…”

Mechanisms for the incorporation of proteins in membranes and organelles.

“…The apparent unrestricted capacity of microsomal membranes from dog pancreas (16) HeLa cells (112), ascites tumor cells (138,225), and hen oviduct (229) to effect, in vitro, the vectorial transfer, removal of signal peptides, and cotranslational glycosylation of a wide variety of presecretory polypeptides from different species and cell types indicates that different signal segments are functionally equivalent and probably participate in a single translocation mechanism that has been conserved during evoL lution. Because comparison of the amino acid sequences of signal segments ofnumerous presecretory proteins fails to show a strong sequence homology, it has been concluded (94,101,114,218) that conformational features of the signals, rather than specific primary sequence information, are the functionally important elements that may be recognized by specific receptors in ER membranes . Different signal segments resemble each other in that they contain middle regions of hydrophobicity, and frequently one or two charged residues are present near the amino-terminal end.…”

“…Many signal segments of different presecretory proteins have been sequenced (see 112,114,218). Although their amino acid composition is variable, all signal sequences are characterized by a high proportion ofhydrophobic amino acids .…”