Yeast Kre1p is GPI-anchored and involved in both cell wall assembly and architecture

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Abstract
Kre1p is a cell surface O-glycoprotein involved in a late stage of 1,6-β-glucan formation in the yeast Saccharomyces cerevisiae. Disruption of KRE1 leads to a 40 % reduction in the overall 1,6-β-glucan content of the cell wall. This paper shows that in a yeast Δkre1 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 Δkre1 null mutation and restores cell wall 1,6-β-glucan content up to wild-type level. In a yeast gpi1 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 β-d-glucan, the amount of cell wall mannoproteins is not significantly decreased in a Δkre1 disruptant, as could be confirmed in competition assays by investigating toxin binding to isolated cell wall mannoproteins. Since the yeast Δkre1 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 Δkre1 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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