title: Initiation and processing in vitro of the primary translation products of guinea-pig caseins.
abstract: 1. Guinea-pig caseins synthesized in a mRNA-directed wheat-germ cell-free protein-synthesizing system represent the primary translation products, even though they appear to be of lower molecular weight when analysed by sodium dodecyl sulphate/polyacrylamide-gel electrophoresis in parallel with caseins isolated from guinea-pig milk. 2. Identification of the N-terminal dipeptide of the primary translational product of caseins A, B and C and alpha-lactalbumin showed that all shared a common sequence, which was identified as either Met-Arg or Met-Lys. 3. Procedures utilizing methionyl-tRNAfMet or methionyl-tRNAMet in the presence or absence of microsomal membranes during translation provide a rapid method of distinguishing between N-terminal processing of peptides synthesized in vitro and other post-translational modifications (glycosylation, phosphorylation), which also result in a change in mobility of peptides when analysed by sodium dodecyl sulphate/polyacrylamide-gel electrophoresis. 4. The results demonstrate that guinea-pig caseins, in common with most other secretory proteins, are synthesized with transient N-terminal 'signal'-peptide extensions, which are cleaved during synthesis in the presence of microsomal membranes.
mesh_terms: D000818:Animals; D002364:Caseins / Q000096:biosynthesis* / Q000235:genetics; D002474:Cell-Free System; D004591:Electrophoresis, Polyacrylamide Gel; D005260:Female; D006168:Guinea Pigs; D066298:In Vitro Techniques; D007425:Intracellular Membranes / Q000378:metabolism; D008715:Methionine / Q000378:metabolism; D008861:Microsomes / Q000378:metabolism; D009239:N-Formylmethionine / Q000378:metabolism; D010442:Peptide Chain Initiation, Translational; D014176:Protein Biosynthesis; D012343:RNA, Transfer / Q000378:metabolism
chemical_list: D002364:Caseins; D009239:N-Formylmethionine; D012343:RNA, Transfer; D008715:Methionine