The peroxin Pex19p interacts with multiple, integral membrane proteins at the peroxisomal membrane

Pex19p is a protein required for the early stages of peroxisome biogenesis, but its precise function and site of action are unknown. We tested the interaction between Pex19p and all known Pichia pastoris Pex proteins by the yeast two-hybrid assay. Pex19p interacted with six of seven known integral peroxisomal membrane proteins (iPMPs), and these interactions were confirmed by coimmunoprecipitation. The interactions were not reduced upon inhibition of new protein synthesis, suggesting that they occur with preexisting, and not newly synthesized, pools of iPMPs. By mapping the domains in six iPMPs that interact with Pex19p and the iPMP sequences responsible for targeting to the peroxisome membrane (mPTSs), we found the majority of these sites do not overlap. Coimmunoprecipitation of Pex19p from fractions that contain peroxisomes or cytosol revealed that the interactions between predominantly cytosolic Pex19p and the iPMPs occur in the organelle pellet that contains peroxisomes. These data, taken together, suggest that Pex19p may have a chaperone-like role at the peroxisome membrane and that it is not the receptor for targeting of iPMPs to the peroxisome.