PXA domain-containing protein Pxa1 is required for normal vacuole function and morphology in Schizosaccharomyces pombe

PhoX homology (PX) domain-containing proteins play critical roles in vesicular trafficking, protein sorting, and lipid modification in eukaryotic cells. Several proteins with PX domains contain an associated domain termed PXA (PX-associated). Although PXA domain-containing proteins are required for some important cellular processes, the function of the PXA domain is unknown. We identified three PXA domain-containing proteins in Schizosaccharomyces pombe. S. pombe Pxa1p (SPAC5D6.07c) contained only the PXA domain, not the PX domain. To elucidate the role of the PXA domain in eukaryotic cells, we constructed and characterized a disruption mutant, pxa1. The pxa1 disruptant contained enlarged vacuoles and exhibited mislocalization of vacuolar carboxypeptidase Y (CPY). The conversion rate from pro- to mature-CPY was greatly impaired in pxa1 cells, and fluorescence microscopy indicated that a sorting receptor for CPY, Vps10p, mislocalized to the vacuolar membrane. The mutants were also deficient in vacuolar sorting of a multivesicular body (MVB) marker, a ubiquitin-GFP- carboxypeptidase S (Ub-GFP-CPS) fusion protein. Taken together, these results indicate that Pxa1 protein is required for normal vacuole function and morphology in S. pombe.