Abstract
The Saccharomyces diastaticus glucoamylase encoded by STA1 contains two signal sequences for potent secretion of the enzyme, a hydrophobic leader peptide (HL), and a tract consisting of threonineand serine-rich sequences (TS); hybrid proteins of Escherichia coli β-galactosidase carrying both HL and TS are secreted through the cytoplasmic membrane to the cell-surface fraction of yeast cells, but those carrying either HL or TS are not. To investigate the molecular mechanisms for these signal sequences, we have isolated a dominant mutation, SSD1, which suppresses a secretory defect caused by deletion of these sequences. Yeast cells harboring the mutation secreted hybrid β-galactosidase proteins carrying either HL or TS into the cell-surface fraction. Even β-galactosidase itself was secreted to the cell surface in the mutant. These results suggest that HL and TS interact with a wild-type ssd1+ gene product to promote protein secretion.
