Kre1p is a cell surface -glycoprotein involved in a late stage of 1,6--glucan formation in the yeast . Disruption of leads to a 40 % reduction in the overall 1,6--glucan content of the cell wall. This paper shows that in a yeast Δ null mutant, neither an N-terminal-truncated Kre1p nor Kre1p variants lacking a C-terminal glycosylphospatidylinositol (GPI) attachment site are capable of achieving normal function in glucan assembly, while full-length Kre1p completely complements a Δ null mutation and restores cell wall 1,6--glucan content up to wild-type level. In a yeast mutant, a green-fluorescent-protein-tagged Kre1p derivative is secreted into the medium, indicating an at least transient GPI-anchoring stage of Kre1p during its processing within the yeast secretory pathway. In contrast to the severe defect in cell wall --glucan, the amount of cell wall mannoproteins is not significantly decreased in a Δ disruptant, as could be confirmed in competition assays by investigating toxin binding to isolated cell wall mannoproteins. Since the yeast Δ mutant differed in its sensitivity to zygocin and K28, two killer viral protein toxins that use different cell wall mannoprotein populations as a primary toxin receptor, it can be concluded that in a yeast Δ background, mannoproteins do not differ significantly in total amount from a Kre1 wild-type but rather in their expression and distribution at the cell surface. Taken together, these data favour and suggest a structural, rather than enzymic, function of Kre1p in yeast cell wall assembly.


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