Nonvesicular phospholipid transfer between peroxisomes and the endoplasmic reticulum.

The endoplasmic reticulum (ER) plays an important role in peroxisome biogenesis; some peroxisomal membrane proteins are inserted into the ER and trafficked to peroxisomes in vesicles. These vesicles could also provide the phospholipids required for the growth of peroxisomal membranes, because peroxisomes lack phospholipid biosynthesis enzymes. To test this, we established a novel assay to monitor phospholipid transfer between the ER and peroxisomes and found that phospholipids are rapidly trafficked between these compartments. This transport is not blocked in mutants with conditional defects in Sec proteins required for vesicular trafficking from the ER or in Pex3p, a protein required for peroxisome membrane biogenesis. ER to peroxisome lipid transport was reconstituted in vitro and does not require cytosolic factors or ATP. Our findings indicate that lipids are directly transferred from the ER to peroxisomes by a nonvesicular pathway and suggest that ER to peroxisome vesicular transport is not required to provide lipids for peroxisomal growth.