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Mol. Cell. Biol., 10 1995, 5671-5681, Vol 15, No. 10
P Mazur, N Morin, W Baginsky, M el-Sherbeini, JA Clemas, JB Nielsen and F Foor
1,3-beta-D-Glucan is a major structural polymer of yeast and fungal cell
walls and is synthesized from UDP-glucose by the multisubunit enzyme
1,3-beta-D-glucan synthase. Previous work has shown that the FKS1 gene
encodes a 215-kDa integral membrane protein (Fks1p) which mediates
sensitivity to the echinocandin class of antifungal glucan synthase
inhibitors and is a subunit of this enzyme. We have cloned and sequenced
FKS2, a homolog of FKS1 encoding a 217-kDa integral membrane protein
(Fks2p) which is 88% identical to Fks1p. The residual glucan synthase
activity present in strains with deletions of fks1 is (i) immunodepleted by
antibodies prepared against FKS2 peptides, demonstrating that Fks2p is also
a component of the enzyme, and (ii) more sensitive to the echinocandin
L-733,560, explaining the increased sensitivity of fks1 null mutants to
this drug. Simultaneous disruption of FKS1 and FKS2 is lethal, suggesting
that Fks1p and Fks2p are alternative subunits with essential overlapping
function. Analysis of FKS1 and FKS2 expression reveals that transcription
of FKS1 is regulated in the cell cycle and predominates during growth on
glucose, while FKS2 is expressed in the absence of glucose. FKS2 is
essential for sporulation, a process which occurs during nutritional
starvation. FKS2 is induced by the addition of Ca2+ to the growth medium,
and this induction is completely dependent on the Ca2+/calmodulin-dependent
phosphoprotein phosphatase calcineurin. We have previously shown that
growth of fks1 null mutants is highly sensitive to the calcineurin
inhibitors FK506 and cyclosporin A. Expression of FKS2 from the
heterologous ADH1 promoter results in FK506-resistant growth. Thus, the
sensitivity of fks1 mutants to these drugs can be explained by the
calcineurin-dependent transcription of FKS2. Moreover, FKS2 is also highly
induced in response to pheromone in a calcineurin-dependent manner,
suggesting that FKS2 may also play a role in the remodeling of the cell
wall during the mating process.
Copyright © 1995, American Society for Microbiology
Differential expression and function of two homologous subunits of yeast 1,3-beta-D-glucan synthase
Merck Research Laboratories, Rahway, New Jersey 07065, USA.
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