Molecular mechanisms of import of peroxisome-targeting signal type 2 (PTS2) proteins by PTS2 receptor Pex7p and PTS1 receptor Pex5pL.

In the present study, we investigated molecular mechanisms underlying the import of peroxisome-targeting signal type 2 (PTS2) proteins into peroxisomes. Purified Chinese hamster Pex7p that had been expressed in an Sf9/baculovirus system was biologically active in several assays such as those for PTS2 binding and assessing the restoration of the impaired PTS2 protein import in Chinese hamster ovary (CHO) pex7 mutant ZPG207. Pex7p was eluted as a monomer in gel filtration chromatography. Moreover, the mutation of the highly conserved cysteine residue suggested to be involved in the dimer formation did not affect the complementing activity in ZPG207 cells. Together, Pex7p more likely functions as a monomer. Together with PTS1 protein, the Pex7p-PTS2 protein complex was bound to Pex5pL, the longer form of Pex5p, which was prerequisite for the translocation of Pex7p-PTS2 protein complexes. Pex5pL-(Pex7p-PTS2 protein) complexes were detectable in wild-type CHO-K1 cells and were apparently more stable in pex14 CHO cells deficient in the entry site of the matrix proteins, whereas only the Pex7p-PTS2 protein complex was discernible in a Pex5pL-defective pex5 CHO mutant. Pex7p-PTS2 proteins bound to Pex14p via Pex5pL. In contrast, PTS2 protein-bound Pex7p as well as Pex7p directly and equally interacted with Pex13p, implying that the PTS2 cargo may be released at Pex13p. Furthermore, we detected the Pex13p complexes likewise formed with Pex5pL-bound Pex7p-PTS2 proteins. Thus, the Pex7p-mediated PTS2 protein import shares most of the steps with the Pex5p-dependent PTS1 import machinery but is likely distinct at the cargo-releasing stage.