Structure of Pex8 in complex with peroxisomal receptor Pex5 reveals its essential role in peroxisomal cargo translocation

Peroxisomes are essential cellular organelles that enable the sequestered execution of a broad range of metabolic processes. Due to the lack of an internal protein synthesis machinery, they entirely depend on the import of target proteins to carry out their functions within peroxisomes. While the process of cargo/receptor recognition is well understood, knowledge about the molecular mechanisms of the subsequent translocation steps, including cargo release and receptor recycling, is lacking behind. Here, we provide structural and functional evidence on the role of Pex8 in these processes. First, we show that Pex8 in yeast is essential for peroxisomal cargo translocation, irrespective of the mechanism of receptor/cargo recognition. Next, we reveal that Pex8 binds through an irregular twelvefold HEAT repeat array to a short three-helical bundle within the otherwise unfolded N-terminal domain of the Pex5 receptor. Impairing this interaction abolishes peroxisomal protein translocation. It is complemented by a secondary autonomous Pex8 cargo-like interaction site with the C-terminal domain of Pex5, thus generating a bipartite interaction between the two proteins. Our data support a model in which Pex5/Pex8 complex formation allows assembly with the peroxisomal Pex2/Pex10/Pex12 E3-ubiquitin ligase complex to initiate recycling of the receptor. In summary, our findings provide in-depth insight into the transition from cargo release into peroxisomes to receptor recycling, which is essential to uncover the overall process of the peroxisomal cargo translocation.