Differential proteolytic sensitivity of yeast fatty acid synthetase subunits α and β contributing to a balanced ratio of both fatty acid synthetase components

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Abstract
The Saccharomyces cerevisiae genes FAS1 and FAS2 encoding the β and α subunit of yeast fatty acid synthetase (FAS), respectively, were individually deleted by one‐step gene disruption. Northern blot analysis of RNA from the resulting fas null allele mutants indicated that deletion of FAS2 did not influence the transcription of FAS1, while FAS2 transcription was significantly reduced in the Δfas1 strain. These data suggest an activating role of subunit β on FAS2 gene expression or, alternatively, a repression of FAS2 by an excess of its own gene product. Compared to the intact α6β6 complex, the individual FAS subunits synthesized in the Δfas1 or Δfas2 strains exhibit a considerably increased sensitivity towards the proteinases present in the yeast cell homogenate. Using yeast mutants specifically defective in the vacuolar proteinases yscA (PRA1/PEP4 gene product) and/or yscB (PRB1 gene product), it was shown that in vitro, subunit α is efficiently degraded by proteinase yscA while for degradation of subunit β, the combined action of proteinases yscA and yscB is necessary. In vivo, besides the vacuolar proteinases, an additional proteolytic activity specifically affecting free FAS subunit α becomes increasingly apparent in cells entering the stationary growth phase. In contrast, under similar conditions uncomplexed FAS subunit β is stable in strains lacking the vacuolar proteinases yscA and yscB. The reduced FAS subunit levels, at the stationary phase, were independent of the corresponding FAS transcript concentrations. Thus, differential degradation pathways are obviously removing an excess of either FAS subunit, at least under starvation conditions. A combination of both regulation of FAS gene expression and proteolysis of free FAS polypeptides may therefore explain the equimolar amounts of both FAS subunits observed in yeast wild‐type cells.