Structure and Function of the Peroxisomal Translocon
Protein import into peroxisomes differs from protein import into other organelles in two aspects: 1) Peroxisomes import folded even oligomeric protein in an unknown manner and 2) the peroxisomal import receptors cycle between a soluble free form in the cytosol and a cargo-loaded form tightly associated with the peroxisomal membrane. With the intention to disclose a novel mechanism of protein transport across membranes, this consortium aims to elucidate the structure and function of the translocon of the peroxisomal protein import machinery.
Peroxisomal matrix proteins contain specific peroxisomal targeting signals (PTS1 or PTS2) that are post-translationally recognized and bound in the cytosol by the peroxisomal import receptors, which direct the receptor-cargo complex to the peroxisomal membrane. The cargo-loaded receptors insert into the peroxisomal membrane and assemble with other membrane proteins to form the translocon, which as a transient pore allows the translocation of the folded proteins across the membrane.
The research group focuses on the:
• Structure and associated dynamics of the peroxisomal translocon for PTS1-proteins
• Analysis of membrane insertion and pore formation of the import receptors
• Elucidation of the mechanism of the translocation of folded protein across the peroxisomal membrane
• Regulation of the function of the translocon
• Characterization of the translocon for PTS2-proteins
Figure 1: Peroxisomal matrix protein import in S. cerevisiae. The peroxisomal matrix protein import is based on soluble receptors, which shuttle between the cytosol and the peroxisomal membrane, thereby guiding cargo proteins to the peroxisomal translocation machinery. (I) The receptor Pex5p recognizes matrix proteins via their PTS1 in the cytosol, and (II) ferries them to the docking complex (Pex13p, Pex14p, Pex17p) at the peroxisomal membrane. Assembly of the cargo-loaded Pex5p with the docking complex is supposed to result in the formation of a transient pore, whose exact molecular organization is still under discussion but at least contains Pex5p and Pex14p. (III) The cargo is translocated into the peroxisomal lumen in an unknown manner, possibly remaining associated with its receptor. Then the receptor-cargo complex dissociates and the cargo is released into the peroxisomal lumen, a process which possibly involves Pex8p. (IV) Monoubiquitination of the receptor is accomplished by the Pex22p-anchored ubiquitin-conjugating Pex4p in concert with the ubiquitin ligase Pex12p, which forms the RING complex together with the other ubiquitin ligases Pex2p and Pex10p. The ubiquitin modification serves as signal for the ATP-dependent dislocation of Pex5p from the peroxisomal membrane back to the cytosol. This process is performed by the AAA-peroxins Pex1p and Pex6p, which are anchored to the peroxisomal membrane via Pex15p. Removal of the ubiquitin is supposed to complete the receptor cycle and provides Pex5p for new rounds of import. Grimm et al., 2012