INITIATION AND PROCESSING INVITRO OF THE PRIMARY TRANSLATION PRODUCTS OF GUINEA-PIG CASEINS

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 MW when analyzed by sodium dodecyl sulfate/polyacrylamide-gel electrophoresis in parallel with caseins isolated from guinea pig milk. 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. Procedures utilizing methionyl-tRNAfMet or methionyl-tRNAMet in the presence or absence of [dog pancreas] 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 analyzed by sodium dodecyl sulfate/polyacrylamide-gel electrophoresis. 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.