Pas10 and Pas20 are two genes essential for peroxisomal protein import in S. cerevisiae. The Pas10 protein is mainly located in the cytosol, with a small fraction associated to the peroxisome. The Pas20 protein is a peroxisomal membrane protein and contains a Src-homology (SH3) domain in its C-terminal region. In a yeast two-hybrid screen Pas10 interacts with the Pas20-SH3 domain. A ligand blot assay demonstrated that this interaction is direct. We propose that Pas10p functions as a mobile receptor, which selects proteins containing a type I peroxisomal targeting signal (PTS1) in the cytosol and docks via the SH3 domain of Pas20p at the peroxisomal import sites. To test this model, we want to reconstitute the PTS1p/Pas10p/Pas20p complex in vitro and measure the interactions, in real time, using a BIAcore apparatus. We purified Pas10p and Pas20-SH3 proteins from E.coli expression systems and prepared synthetic peptides containing a functional PTS1-signal. By coating the BIAsensor chip with Pas20-SH3 protein, the affinity and kinetics of the Pas10p-Pas20p interaction in relation to the presence of PTS1-peptides can be analysed.