FIGURE 1: Current PMP sorting models.

(A) PMP sorting requires the Pex3/Pex9 sorting machinery. In this classical model Pex19 functions as receptor/chaperone to sort newly synthesized PMP molecules to the peroxisomal docking site Pex3 followed by PMP insertion in the membrane by yet unknown mechanisms.

(B) PMP sorting requires the ER. According to this model peroxisomes are formed from the ER. PMPs are first inserted in the ER dependent of the Sec61 machinery and subsequently incorporated in single or multiple vesicles that are separated from the ER via the function of Pex3 and Pex19. In case of multiple vesicles, these fuse to form a pre-peroxisomal vesicle.