Trypanosoma bruceiPex13.2 is an accessory peroxin that functions in the import of PTS2 proteins and localizes to subdomains of the glycosome

Abstract

Kinetoplastid parasites including Trypanosoma brucei, Trypanosoma cruzi and Leishmania harbor unique organelles known as glycosomes, which are evolutionarily related to peroxisomes. Glycosome/peroxisome biogenesis is mediated by proteins called peroxins that facilitate organelle formation, proliferation and degradation, and import of proteins housed therein. Import of matrix proteins occurs via one of two pathways that are dictated by their peroxisome targeting sequence (PTS). In PTS1 import, a C-terminal tripeptide sequence, most commonly SKL, is recognized by the soluble receptor Pex5. In PTS2 import, a less conserved N-terminal sequence is recognized by Pex7. The soluble receptors deliver their cargo to the import channel consisting minimally of Pex13 and Pex14. While much of the import process is conserved, kinetoplastids are the only organisms to have two Pex13s, TbPex13.1 and TbPex13.2. In previous studies, GFP-tagged TbPex13.1 localized to glycosomes and silencing either protein in the stage of the parasite that lives in the mammalian bloodstream impaired glycosome protein import and slowed parasite growth. While these findings suggest Pex13s are involved in protein import, the mechanisms by which they function are unknown and it is unclear why kinetoplastids would require two Pex13s. In this work, we demonstrate that TbPex13.2 is associated with the glycosome membrane with its N-terminus facing the cytoplasm. Super-resolution microscopy reveals that TbPex13.2 localizes to a few (1-3) foci per glycosome and import of PTS2 proteins was disrupted in TbPex13.2-deficient cells suggesting it may be an accessory factor for PTS2 import.