Pex12p of Saccharomyces cerevisiae is a component of a multi-protein complex essential for peroxisomal matrix protein import

We have isolated the Saccharomyces cerevisiae pex12-1 mutant from a screen to identify mutants defective in peroxisome biogenesis, The pex12 Delta del etion strain fails to import peroxisomal matrix proteins through both the P TS1 and PTS2 pathway. The PEX12 gene was cloned by functional complementati on of the pex12-1 mutant strain and encodes a polypeptide of 399 amino acid s. ScPex12p is orthologous to Pex12 proteins from other species and like it s orthologues, S, cerevisiae Pex12p contains a degenerate RING finger domai n of the C3HC4 type in its essential carboxy-terminus. Localization studies demonstrate that Pex12p is an integral peroxisomal membrane protein, with its NH2-terminus facing the peroxisomal lumen and with its COOH-terminus fa cing the cytosol, Pex12p - deficient cells retain particular structures tha t contain peroxisomal membrane proteins consistent with the existence of pe roxisomal membrane remnants ("ghosts") in pex12 Delta null mutant cells. Th is finding indicates that pex12 Delta cells are not impaired in peroxisomal membrane biogenesis. In immunoisolation experiments Pex12p was co-purified with the RING finger protein Pex10p, the PTS1 receptor Pex5p and the docki ng proteins for the PTS1 and the PTS2 receptor at the peroxisomal membrane, Pex13p and Pex14p, Furthermore, two-hybrid experiments suggest that the tw o RING finger domains are sufficient for the Pex1op-Pex12p interaction. Our results suggest that Pex12p is a component of the peroxisomal translocatio n machinery for matrix proteins.