Involvement of Pex13p in Pex14p localization and peroxisomal targeting signal 2-dependent protein import into peroxisomes

Pex13p is the putative docking protein for peroxisomal targeting signal 1 ( PTS1)-dependent protein import into peroxisomes. Pex14p interacts with both the PTS1- and PTS2-receptor and may represent the paint of convergence of the PTS1- and PTS2-dependent protein import pathways. We report the involve ment of Pex13p in peroxisomal import of PTS2-containing proteins. Like Pex1 4p, Pex13p not only interacts with the PTS1-receptor Pex5p, but also with t he PTS2-receptor Pex7p; however, this association may be direct or indirect . In support of distinct peroxisomal binding sites for Pex7p, the Pex7p/Pex 13p and Pex7p/Pex14p complexes can form independently. Genetic evidence for the interaction of Pex7p and Pex13p is provided by the observation that ov erexpression of Pex13p suppresses a loss of function mutant of Pex7p, Accor dingly, we conclude that Pex7p and Pex13p functionally interact during PTS2 -dependent protein import into peroxisomes. NH2-terminal regions of Pex13p are required for its interaction with the PTS2-receptor while the COOH-term inal SH3 domain alone is sufficient to mediate its interaction with the PTS 1-receptor. Reinvestigation of the topology revealed both termini of Pex13p to be oriented towards the cytosol. We also found Pex13p to be required fo r peroxisomal association of Pex14p, yet the SH3 domain of Pex13p may not p rovide the only binding site for Pex14p at the peroxisomal membrane.