Localization of Phr1p to the septum and glucan remodeling in Candida albicans

PHR1 belongs to a family of five genes of C. albicans (PHR1-3, PGA4 and PGA5). PHR1 expression is triggered at external pH values > 5.5. Phr1p is endowed with β(1,3)-glucan elongase activity (GH72) similarly to Gas1p of S. cerevisiae and Gel proteins of A. fumigatus. At the restrictive pH, loss of PHR1 induces morphological defects, a compensatory increase of chitin, inability to support hyphal growth, to adhere and invade human epithelia. PHR1 mutants are avirulent in mouse models of systemic infection. By use of a GFP-Phr1 hybrid protein, we detected the protein preferentially 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 appearing as a bright line. These results indicate that Phr1p localizes preferentially to sites of active wall formation. We further studied the localization of Phr1p in the septum region by analyzing cells lacking Chs3p, that synthesizes the chitin ring at the rim of the primary septum. Differently from ScGas1p, that is cross-linked to the chitin ring and remains in the bud scars even after several generations, Phr1p-GFP did not persist indicating that the mechanisms active in tethering Gas1p to the bud scars in S. cerevisiae are not operative in C. albicans. Further, Phr1p-GFP was localized at/or in close proximity of the primary septum indicating that it could be required for glucan remodeling of the secondary septa. Further, cytostatic concentrations of a highly specific inhibitor of the activity of Chs1p, essential for the synthesis of the primary septum and for cell integrity, proved to be fungicidal in the absence of Phr1p. The results of these studies and the detection of defects in nuclei segregation, suggest that Phr1p and Chs1p synergically cooperate in maintaining cell integrity and coupling morphogenesis with nuclear division.