Pmt1 mannosyl transferase is involved in cell wall incorporation of several proteins in Saccharomyces cerevisiae

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Abstract
We constructed hybrid proteins containing a plant α‐galactosidase fused to various C‐terminal moieties of the hypoxic Srp1p; this allowed us to identify a cell wall‐bound form of Srp1p. We showed that the last 30 amino acids of Srp1p, but not the last 16, contain sufficient information to signal glycosyl‐phosphatidylinositol anchor attachment and subsequent cell wall anchorage. The cell wall‐bound form was shown to be linked by means of a β1,6‐glucose‐containing side‐chain. Pmt1p enzyme is known as a protein‐O‐mannosyltransferase that initiates the O‐glycosidic chains on proteins. We found that a pmt1 deletion mutant was highly sensitive to zymolyase and that in this strain the α‐galactosidase–Srp1 fusion proteins, an α‐galactosidase–Sed1 hybrid protein and an α‐galactosidase–α‐agglutinin hybrid protein were absent from both the membrane and the cell wall fractions. However, the plasma membrane protein Gas1p still receives its glycosyl‐phosphatidylinositol anchor in pmt1 cells, and in this mutant strain an α‐galactosidase–Cwp2 fusion protein was found linked to the cell wall but devoid of β1,6‐glucan side‐chain, indicating an alternative mechanism of cell wall anchorage.