Sxa2 is a serine carboxypeptidase that degrades extracellular P-factor in the fission yeast Schizosaccharomyces pombe.

Abstract

Stimulating the fission yeast Schizosaccharomyces pombe with mating pheromones brings about responses that lead to cell conjugation. Persistent stimulation does not, however, induce a continuous response as the cells become desensitized to the presence of the pheromone. One mechanism that contributes to desensitization in M-cells is the release of a carboxypeptidase that inactivates the extracellular P-factor pheromone. Production of the carboxypeptidase requires a functional sxa2 gene. In this study, we report the first molecular characterization of the Sxa2 protein and provide direct evidence that it is the carboxypeptidase that degrades P-factor. Sxa2 is synthesized as a precursor that undergoes an internal cleavage event catalysed by a protease with specificity for basic residues. This generates a series of catalytically active N-terminal fragments and an inactive C-terminal fragment. Cleavage is essential for activation of the carboxypeptidase and, although the C-terminal fragment is inactive, it is required for the N-terminal fragment to attain activity.