vga Mutants of Kluyveromyces lactis show cell integrity defects.

We studied the cell wall alterations that occur in mutants of Kluyveromyces lactis impaired in glycosylation. The mutants belong to four complementation groups named vga1 to vga4 (vanadate glycosylation affected), characterized by sodium orthovanadate resistance and alteration of the glycosylation profile of native invertase. A drastic reduction of the alkali-soluble fraction of the beta-D-glucan was observed in vga1, vga2 and vga3 cells, accompanied by an increase in the chitin content of the cell wall. In vga4 cells, both beta-D-glucan fractions (alkali-soluble and alkali-insoluble) were reduced to about half of the corresponding wild-type value but the chitin content was normal. A protein related to Fks1p, the catalytic subunit of the major 1,3-beta-D-glucan synthase of S. cerevisiae, was detected in K. lactis. The amount of this Fks1p-like protein increased 7-10 times in vga1, vga2 and vga3 mutants as compared to wild-type cells; the same strains released significant amounts of beta-D-glucan in the culture supernatant. These mutations also resulted in abnormally thick cell walls with conspicuous irregularities in the structure, as revealed by electron microscopy and by an altered resistance to Zymolyase. The observed high responsiveness of cell wall phenotypes to alterations of glycosylation make K. lactis an attractive system for studying the interconnections between these processes. Copyright 2000 John Wiley & Sons, Ltd.