FIGURE 7.

Model of yeast PTS2-dependent protein import into peroxisomes. The Fox3p (thiolase)-loaded PTS2-receptor complex consisting of Pex7p and Pex18p associates with the peroxisomal docking complex (1). Pex18p is supposed to insert into the membrane, whereas Pex7p remains on the cytosolic face of the peroxisomal compartment (2). Next, Pex18p as well as Pex7p change their topology. While Pex18p becomes accessible to externally added proteases, Pex7p gains a protected state, which might indicate translocation of the cargo-loaded receptor across the membrane. This change in topology depends on ubiquitination of the conserved cysteine residue in the N-terminal domain of Pex18p (3). Finally, after the cargo has been released into the peroxisomal lumen, Pex7p as well as Pex18p are transported back to the cytosol (4).