Correspondence

The yeast cell wall contains an unusually high number of different mannoproteins. The physiological role of most of them is unknown and gene disruptions leading to depletion of different proteins do not affect major functions of the wall. In this work the phenotype of different single and multiple cell wall protein mutants was observed at the level of individual cells. It was found that the lack of the non-covalently bound wall proteins Scw4p, Scw10p and Bgl2p increases the mortality of Saccharomyces cerevisiae cells grown exponentially under standard laboratory conditions, as assayed by methylene blue staining. Mutation of SCW11, however, suppressed the phenotype of scw4scw10, or scw4scw10bgl2, indicating that Scw4p, Scw10p and Bgl2p act synergistically while Scw11p has an activity antagonistic to that of the other three proteins. Mutants lacking major covalently bound proteins, either all four described Pir-proteins or the five most abundant glycosylphosphatidylinositol (GPI)-anchored proteins (Ccw12p, Ccw13p/Dan1p, Ccw14p/Icwp1p, Tip1p and Cwp1p), also had increased mortalities, the first somewhat more and the latter less than that of scw4scw10bgl2. In all cases the observed phenotype was suppressed by the addition of an osmotic stabilizer to the growth medium, indicating that cells died due to decreased osmotic stability. If cells were grown to stationary phase, Scw-mutants showed only slightly increased mortality, but mutants lacking Pir- or GPI-anchored proteins had significantly increased sensitivity, suggesting that their physiological function is primarily expressed in stationary-phase cells. In many cases structures consisting of a living ccw5ccw6ccw7ccw8 (multiple Pir-protein mutant) mother with two methylene blue-stained daughters could be seen