THE SH3 DOMAIN OF THE SACCHAROMYCES-CEREVISIAE PEROXISOMAL MEMBRANE-PROTEIN PEX13P FUNCTIONS AS A DOCKING SITE FOR PEX5P, A MOBILE RECEPTORFOR THE IMPORT OF PTS1-CONTAINING PROTEINS

We identified a Sacchnromyces cerevisiae peroxisomal membrane protein, Pex13p, that is essential for protein import. A point mutation in the COOH-terminal Src homology 3 (SH3) domain of Pex13p inactivated the p rotein but did not affect its membrane targeting. A two-hybrid screen with the SH3 domain of Pex13p identified Pex5p, a receptor for protein s with a type I peroxisomal targeting signal (PTS1), as its ligand. Pe x13p SH3 interacted specifically with Pex5p in vitro. We determined, f urthermore, that Pex5p was mainly present in the cytosol and only a sm all fraction was associated with peroxisomes. We therefore propose tha t Pex13p is a component of the peroxisomal protein import machinery on to which the mobile Pex5p receptor docks for the delivery of the selec ted PTS1 protein.