Journal of Bacteriology, November 1999, p. 7070-7079, Vol. 181, No. 22
0021-9193/99/$04.00+0
Copyright © 1999, American Society for Microbiology. All rights reserved.
-1,3- and
-1,6-Glucans
Department of Microbiology and Immunology, Georgetown University, Washington, D.C. 20007-2197
Received 29 June 1999/Accepted 7 September 1999
PHR1 and PHR2 encode putative
glycosylphosphatidylinositol-anchored cell surface proteins of the
opportunistic fungal pathogen Candida albicans. These
proteins are functionally related, and their expression is modulated in
relation to the pH of the ambient environment in vitro and in vivo.
Deletion of either gene results in a pH-conditional defect in cell
morphology and virulence. Multiple sequence alignments demonstrated a
distant relationship between the Phr proteins and
-galactosidases.
Based on this alignment, site-directed mutagenesis of the putative
active-site residues of Phr1p and Phr2p was conducted and two conserved
glutamate residues were shown to be essential for activity. By taking
advantage of the pH-conditional expression of the genes, a temporal
analysis of cell wall changes was performed following a shift of the
mutants from permissive to nonpermissive pH. The mutations did not
grossly affect the amount of polysaccharides in the wall but did alter their distribution. The most immediate alteration to occur was a
fivefold increase in the rate of cross-linking between
-1,6-glycosylated mannoproteins and chitin. This increase was
followed shortly thereafter by a decline in
-1,3-glucan-associated
-1,6-glucans and, within several generations, a fivefold increase in
the chitin content of the walls. The increased accumulation of
chitin-linked glucans was not due to a block in subsequent processing
as determined by pulse-chase analysis. Rather, the results suggest that
the glucans are diverted to chitin linkage due to the inability of the
mutants to establish cross-links between
-1,6- and
-1,3-glucans. Based on these and previously published results, it is suggested that
the Phr proteins process
-1,3-glucans and make available acceptor
sites for the attachment of
-1,6-glucans.
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