Abstract

The peroxisome is an organelle known for its essential role in lipid metabolism and hydrogen peroxide detoxification. In the yeast Yarrowia lipolytica , six subforms of peroxisomes, termed P1 to P6, are organized into a multistep peroxisome assembly pathway. The pathway operates by conversion of the subforms in a temporally ordered manner from P1 to P6. The pathway leads to the formation of mature peroxisomes, P6, carrying the complete set of matrix and membrane proteins. The stepwise import of distinct subsets of matrix proteins into immature peroxisomal vesicles P1 to P5 provides them with an increasing fraction of the matrix proteins present in mature peroxisomes P6. This increase in the total mass of matrix proteins above a critical level causes the redistribution of a peroxisomal protein, acyl-CoA oxidase (Aox), from the matrix to the membrane. A significant redistribution of Aox occurs only in mature peroxisomes. Inside mature peroxisomes, the membrane-bound pool of Aox interacts with Pex16p. Pex16p is a membrane-associated protein that negatively regulates the membrane bending, scission and fission events required for the division of immature peroxisomal vesicles, thereby preventing their excessive proliferation. In the inner (lumenal) membrane leaflet of these vesicles, Pex16p binds lysophosphosphatidic acid (LPA). The binding of Pex 16p to LPA prevents the formation of phosphatidic acid (PA) and diacylglycerol (DAG), the two cone-shaped membrane lipids known for their ability to promote the destabilization of the membrane bilayer and to induce strong membrane bending. Only inside mature peroxisomes Pex16p interacts with Aox. This interaction between Aox and Pex16p greatly decreases the affinity between Pex16p and LPA, thereby allowing LPA to enter a two-step biosynthetic pathway leading to the formation of PA and DAG. The resulting accumulation of PA and DAG in the inner (lumenal) leaflet of the membrane of mature peroxisomes and the subsequent transbilayer movement of DAG and the glycerophospholipid phosphatidylserine destabilize and curve the membrane. A complex between the peroxins Pex10p and Pex 19p, the dynamin-like GTPase Vps1p, and several actin cytoskeletal proteins is then assembled on the peroxisomal surface. This protein team promotes membrane scission and fission, thereby executing the terminal steps of peroxisome division.