An a-pheromone transporter Hst6 is involved in pheromone-induced cell adhesion, mating and White-Opaque transition of Candida albicans

Candida albicans is a prevalent opportunistic human fungal pathogen that can cause vital systemic infections in immunocompromised patients. In order to adapt multiple niches, C. albicans develops a unique phenotypic transition between white and opaque phases. The most behavioral difference of these two cell forms is their sexual fertility. Opaque cells are the mating competent form that secrete cell type-specific (a or α) pheromones to activate mating responses in the opposite cell type. However, although white cells are incapable of mating, pheromones released from minority opaque cells could induce biofilm formation and cell adhesion of white cells. The array profiling has shown that MFA1 (encoded with an a-pheromone precursor) and HST6 (encoded with an a-pheromone transporter) in MTLa/a white cells were highly expressed when challenged with α-pheromone. We therefore hypothesized that MFA1 and HST6 are involved in the cell adhesion during the response to α-pheromone. Deletion of the HST6 gene, an a-type specific a-pheromone transporter, in MTLa/a P37005 strain resulted in the significant reduction of numbers in pheromone-induced cell adhesion (5.2 x 107) comparing with those of the wild-type strain (13.4 x 107), while deletion of the MFA1 gene exhibited no role in its cell adhesion. Preliminary studies have identified the transcriptional regulator of pheromone-induced cell adhesion as Cph1. Quantitative RT-PCR also showed that the expression of the HST6 is directly regulated by Cph1, Cek1 (MAPK) and Ste2 (receptor of α-pheromone). Additionally, hst6Δ showed an extremely low mating efficiency (0.004%) comparing to that of the wild-type strain (65.07%), suggesting that deletion of HST6 could cause a-pheromone peptide secretion defects in C. albicans. To understand if Hst6 involved in pheromone-induced cell adhesion also plays a role in conventional biofilm formation, two genes of biofilm-associated regulators were tested. Expression of TEC1 or NDT80 and the dry mass of biofilms between the wild-type strain and hst6Δ showed no significant differences, suggesting that Hst6 is specifically involved in pheromone-induced cell adhesion but not biofilm development. Interestingly, hst6Δ displayed a lower white-to-opaque switching frequency (1.95 ± 2.51%) than those of the wild-type (34.34 ± 20.16%). However, deletion of the HST6 gene in MTLa/a RBY717 strain exhibited no effects on pheromone-induced cell adhesion and white-to-opaque transition, which implied that the functions of Hst6 might be strain-specific. Taken together, our study has demonstrated that Hst6, an a-pheromone transporter in MTLa/a cells, is involved in pheromone-induced cell adhesion, mating and white-to-opaque transition in P37005 strain of C. albicans, but is dispensable for the formation of conventional biofilms.