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

Stimulating the fission yeast Schizosaccharomyces pombe with mating pheromo nes brings about responses that lead to cell conjugation. Persistent stimul ation does not, however, induce a continuous response as the cells become d esensitized to the presence of the pheromone. One mechanism that contribute s to desensitization in M-cells is the release of a carboxypeptidase that i nactivates the extracellular P-factor pheromone. Production of the carboxyp eptidase requires a functional sxa2 gene. In this study, we report the firs t molecular characterization of the Sxa2 protein and provide direct evidenc e that it is the carboxypeptidase that degrades P-factor. Sxa2 is synthesiz ed as a precursor that undergoes an internal cleavage event catalysed by a protease with specificity for basic residues. This generates a series of ca talytically active N-terminal fragments and an inactive C-terminal fragment . Cleavage is essential for activation of the carboxypeptidase and, althoug h the C-terminal fragment is inactive, it is required for the N-terminal fr agment to attain activity.