Candida Infection Biology – fungal armoury, battlefields ... - FINSysB

Poster number: 29
Localization of the Phr1p, a glucan remodeling enzyme
crucial for morphogenesis and virulence in Candida
albicans
G. Degani and L.Popolo
Università degli Studi di Milano, Dipartimento di Scienze Biomolecolari e Biotecnologie, Via
Celoria 26, Milano, Italy
PHR1 is a pH-regulated gene belonging to a family of five genes of C. albicans.
PHR1 expression is triggered at pH values of the growth medium > 5.5. Loss of
PHR1 induces morphological defects at neutral-alkaline ambient pH, inability to
support hyphal growth, to adhere and invade human epithelia. PHR1 mutants are
avirulent in mouse models of systemic infection. At the restrictive pH, cells lacking
Phr1p have an irregular surface and higher and delocalized content of chitin in the
abnormal germ tubes which the mutant produces. Phr1p is a glycoprotein endowed
with beta(1,3)-glucan elongase activity (GH72) similarly to Gas1p of S. cerevisiae
and Gel proteins of A. fumigatus. We obtained a GFP fusion to determine its cellular
localization. During growth in yeast form Phr1p-GFP was predominantly detected
at the bud periphery, bud-neck and septum. During hyphal growth Phr1p-GFP
concentrated at the tip of the germ tubes and gradually distributed along the lateral
side of the hyphae. Phr1p also localized to the septa of the hyphae. By exploiting
the conditional nature of PHR1 and the intrinsic instability of the fluorescent hybrid,
we performed pH shift-down experiments. Differently from the cell wall-bound form
of ScGas1p which is stabilized by cross-links to the chitin ring and remains in the
bud scars after generations, Phr1p-GFP did not persist. Further studies were
performed using mutants lacking CHS3 or members of the CRH family whose
homologues are known to be required for Gas1p immobilization to the bud scar.
Our results suggest that the activity of Phr1p is required at the sites of wall formation
where the incorporation of newly synthesized beta(1,3)-glucan is strictly coupled
to morphogenic events. Moreover, our studies revealed that the mechanisms active
in tethering Gas1p to the bud scars in S. cerevisiae are not operative in C. albicans.
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