Inactivation of the KlPMR1 gene of Kluyveromyces lactis results in defective cell-wall morphogenesis

The P-type Ca2+-ATPases are the transporters responsible for calcium homeos tasis in the cell compartments of eukaryotes. The KlPMR1 gene of Kluyveromy ces lactis encodes a P-type Ca2+-ATPase, which is functionally and structur ally homologous to Pmr1p of Saccharomyces cerevisiae, the calcium pump loca lized in the Golgi membranes. In this work, a novel involvement of KlPmr1p in cell-wall morphogenesis of K. lactis is reported. Klpmr1 Delta cells exh ibited the loss of outer-chain extension in the glycosylation of secreted p roteins. The absence of KlPmr1p resulted in the accumulation of round, larg e cells with an abnormally thick cell wall, as revealed by transmission ele ctron microscopy. The deletant strain also showed a delocalized deposition of chitin in the lateral cell wall accompanied by an unbalanced ratio of in soluble to soluble glucans. These morphological defects were accompanied by the presence of irregularly shaped nuclei and by a DNA content greater tha n 2n. Addition of 10 mM Ca2+ to the medium of the Klpmr1 Delta strain rever sed the chitin-deposition impairment, recovered the alteration to the gluca n ratio and restored a normal thickness of the cell wall. The mutant cells resumed wildtype size, shape and nuclear morphology but the DNA content ind icated the persistence of defects in the co-ordination between DNA replicat ion and cell division. The glycosylation defects were completely unaffected by the calcium supplement. These results indicate that calcium homeostasis controlled by KlPmr1p plays an important role in the cell-wall morphogenes is of K. lactis.