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Functional Characterization of the Pneumocystis carinii Pheromone Receptor STE2

The cellular differentiation of Pneumocystis carinii (PC) from the trophic forms to the multinucleated cyst is critical to understand the pathogenesis of this fungus. Still the mechanism of life cycle regulation is not well known. In fungi closely related to PC, a mitogen‐activated protein kinase (MAPK) cascade regulates these processes. Conjugation between two haploid cells occurs after pheromones bind to their respective pheromone receptors on cells of opposite mating types. We previously reported a functional analysis of the PCSTE3 pheromone receptor of PC, and now we report characteristics of the cognate receptor, PCSTE2. We cloned the full‐length 1065‐bp cDNA for PCSTE2, of which the translated open‐reading frame encodes a protein of 354 amino acids with greatest homology to the fungal pheromone receptors from S. cerevisiae Ste2 and S. pombe Mam2. Computer prediction modeling of PCSTE2 reveals seven transmembrane domains characteristic of fungal pheromone receptors. Similar to the S. cerevisiae alpha factor receptor Ste2, PCSTE2 has three extracellular domains implicated in ligand binding. An antibody made against the extracellular domain of PCSTE2 identified PCSTE2 receptor expression on a subpopulation of PC organisms in infected rat lung tissue but not normal uninfected tissue. We expressed PCSTE2 in S. cerevisiae Ste2 knockout yeast for functional characterization. Our identification of both PCSTE2 and PCSTE3 pheromone receptors may help to elucidate mechanisms of PC life cycle regulation and potential new therapeutics for treating PC pneumonia through alterations of MAPK signaling.