Distinct Repressible MRNAs for Cytoplasmic and Secreted Yeast Invertase Are Encoded by a Single Gene

We have studied regulation of invertase putative structural genes (SUC) in S. cerevisiae and the synthetic relationship between secreted, glycosylated invertase (E.C.3.2.1.26) and the cytoplasmic, nonglycosylated form of the enzyme. Using immunoprecipitation and gel electrophoresis, we have analyzed invertase polypeptides and glycopeptides synthesized in vitro and in vivo. Analysis of size-fractionated mRNA from a SUC2 strain has shown that three mature, catabolite-repressible mRNA species direct the in vitro synthesis of three invertase polypeptides that have differing molecular weights. Two of these polypeptides, P63 and P62 (63 and 62 kd), are larger than the polypeptides of the secreted enzyme and are cotranslationally processed by microsomal membranes in vitro to yield secreted invertase glycopeptides (GP90 and GP87). The smallest polypeptide, P60 (60 kd), which comigrates electrophoretically with cytoplasmic invertase, is not processed. Posttranslationally, a microsomal-membrane detergent extract removes approximately 20 aminoacids from P62 but not from P60. In vitro translations of mRNAs from a genetically confirmed suc3 mutant strain, from the parental SUC3 strain and from derivative meiotic segregants have shown that the three polypeptides (and therefore three mRNA species) are encoded by one gene. Analysis of in vivo radiolabeled invertase from the same SUC3 and suc3 strains has verified that the SUC3 locus contains the structural gene for secreted and cytoplasmic invertase. Through the derepressed synthesis of multiple primary or processed transcripts, the SUC2 and SUC3 genes are regulated to produce multiple invertase polypeptides. The larger two polypeptides appear to be processed and secreted to yield glycosylated invertase, while the smallest remains in the cytoplasm.