A novel vacuolar protein encoded by SSU21 / MCD4 is involved in cell wall integrity in yeast

Using a screening procedure for obtaining yeast strains with enhanced ability to secrete heterologous protein, we have isolated a mutant with alteration of the cell wall structure. This mutant displayed strong decrease in cell wall mannoprotein content, which was not accompanied by decreased glycosylation of secreted proteins. The mutation defines a gene, designated SSU21(identical to previously characterized MCD4), which encodes a novel vacuolar protein. SSU21 is probably connected to the cell integrity protein kinase C-mediated pathway, since ssu21 and pkc1Delta double mutant is synthetic lethal. To our knowledge, this is the first example of a yeast vacuolar protein whose alteration results in a cell wall defect.