Identification and characterization of a Candida albicans mating pheromone

Abstract

Candida albicans, the most prevalent fungal pathogen of humans, has recently been shown to undergo mating. Here we describe a mating pheromone produced by C. albicans alpha cells and show that the gene which encodes it (MFalpha) is required for alpha cells, but not a cells, to mate. We also identify the receptor for this mating pheromone as the product of the STE2 gene and show that this gene is required for the mating of a cells, but not alpha cells. Cells of the a mating type respond to the alpha mating pheromone by producing long polarized projections, similar to those observed in bona fide mating mixtures of C. albicans a and alpha cells. During this process, transcription of approximately 62 genes is induced. Although some of these genes correspond to those induced in Saccharomyces cerevisiae by S. cerevisiae alpha-factor, most are specific to the C. albicans pheromone response. The most surprising class encode cell surface and secreted proteins previously implicated in virulence of C. albicans in a mouse model of disseminated candidiasis. This observation suggests that aspects of cell-cell communication in mating may have been evolutionarily adopted for host-pathogen interactions in C. albicans.

FIG. 1.

Schematic diagram of the proposed C. albicans α-factor pheromone precursor. The conceptually translated protein consists of a hydrophobic leader sequence and three copies of a 13-amino-acid (aa) sequence. For comparison, the two α-factor genes from S. cerevisiae are also shown. The mature S. cerevisiae α-factor peptide is formed by several protein processing steps, including cleavage by the Kex2 protease (37).

FIG. 2.

Response of C. albicans cells to synthetic α-factor. C. albicans a and α cell types in both the white phase (A) and the opaque phase (B) were treated with 10% DMSO or α-factor (10 μg/ml) dissolved in 10% DMSO. Cells were examined after 4 h of incubation at 25°C. The a and α cell types were generated by gene knockouts (e.g., Δa1/α1α2) or by selection on medium containing sorbose (e.g., MTLα/MTLα), as described in Materials and Methods.

FIG. 3.

Time course of the response of a cells to α-factor. Cells were treated with α-factor (10 μg/ml) or mock treated with DMSO and incubated at 25°C. Samples were taken and analyzed for their response to α-factor at 0, 1, 2, and 4 h.

FIG. 4.

The STE2 gene is required for the response of a cells to α-factor. Opaque a cells or opaque Δste2/Δste2 MTLa cells were treated with 10 μg of α-factor per ml and incubated for 4 h at 25°C. The ste2/ste2 mutant showed no response to α-factor.

FIG.5.

Microarray analysis of the response of a and α opaque cells to α-factor. All spots on the microarray that were induced more than threefold in at least two of the time points are shown. In the first time course experiment (EXPT 1), samples were taken 20 min, 1 h, and 4 h after the addition of DMSO (lanes 2 to 4) or α-factor (lanes 5 to 7) to a cells. In the second time course experiment, samples were taken at 1, 2, and 4 h after the addition of DMSO (lanes 9 to 11) or α-factor (lanes 12 to 14). As an additional control, DMSO or α-factor were also added to α cells, and samples were taken after 4 h (lanes 16 and 17, respectively). All samples were normalized to the opaque a cells at the zero time point. The array results clustered into six sets of genes: a- and α-specific genes (including the MTL genes), DMSO-induced or -repressed genes, and α-factor-induced or -repressed genes. The cluster corresponding to α-factor-induced genes is shown enlarged on the right. Selected genes are indicated to the right of the cluster. Note that many genes are represented by multiple spots on the array (the duplicates are not labeled).

FIG. 6.

Pie chart of genes induced by α-factor in C. albicans opaque a cells.