Isolation and Characterization of α‐Amylase Messenger RNA from Bank Vole Parotid Glands

Bank vole saliva contains two glycogen‐precipitable proteins, both of which show affinity for the α‐amylase inhibitor cycloheptaamylose. One of these proteins, amylase, has a molecular weight of 55000, judged from dodecylsulphate/acrylamide gel electrophoresis. The other has an apparent molecular weight of 59000 and has no amylase activity. We report here that tryptic peptide maps as well as amino‐acid composition analyses indicate extensive homology between the two proteins. We have also isolated total poly(A)‐containing mRNA from amylase‐rich bank vole parotid glands. These mRNAs were translated in the presence of [ 35 S]methionine in an mRNA‐dependent cell‐free translation system from rabbit reticulocyte lysate. The radioactive translation products were examined by dodecylsulphate/polyacrylamide gel electrophoresis. Two major translation products with apparent molecular weights of approximately 56500 and 60500, respectively, were further characterized by tryptic peptide analyses. Our data indicate that the 56500‐ M r product is the biosyntetic precursor of amylase, whereas the 60500‐ M r translation product is a precursor of the 59000‐ M r amylase‐like protein. Both precursors appear to contain extra peptide material, presumably as amino‐terminal ‘pre’ or ‘signal’ peptides, in analogy with that found for other precursors of secretory proteins. Thus, amylase and the 59000‐ M r protein, although very similar, are translated from two separate mRNAs. These two messengers sediment in a sucrose gradient at about 17‐S, corresponding to lengths of about 1800 nucleotides.