Lipid droplet and peroxisome biogenesis occur at the same ER subdomains.

Nascent lipid droplet (LD) formation occurs in the endoplasmic reticulum (ER) membrane but it is not known how sites of biogenesis are determined. We previously identified ER domains in S. cerevisiae containing the reticulon homology domain (RHD) protein Pex30 that are regions where preperoxisomal vesicles (PPVs) form. Here， we show that Pex30 domains are also sites where most nascent LDs form. Mature LDs usually remain associated with Pex30 subdomains， and the same Pex30 subdomain can simultaneously associate with a LD and a PPV or peroxisome. We find that in higher eukaryotes multiple C2 domain containing transmembrane protein (MCTP2) is similar to Pex30: it contains an RHD and resides in ER domains where most nascent LD biogenesis occurs and that often associate with peroxisomes. Together， these findings indicate that most LDs and PPVs form and remain associated with conserved ER subdomains， and suggest a link between LD and peroxisome biogenesis.