Accumulation of caveolin in the endoplasmic reticulum redirects the protein to lipid storage droplets

Caveolin-1 is normally localized in plasma membrane caveolae and the Golgi apparatus in mammalian cells. We found three treatments that redirected the protein to lipid storage droplets, identified by staining with the lipophi lic dye Nile red and the marker protein ADRP. Caveolin-1 was targeted to th e droplets when linked to the ER-retrieval sequence, KKSL, generating CaV-K KSL. Cav-Delta N2 an internal deletion mutant, also accumulated in the drop lets, as well as in a Golgi-like structure. Third, incubation of cells with brefeldin A caused caveolin-1 to accumulate in the droplets. This localiza tion persisted after drug washout, showing that caveolin-1 was transported out of the droplets slowly or not at all. Some overexpressed caveolin-2 was also present in lipid droplets. Experimental reduction of cellular cholest eryl ester by 80% did not prevent targeting of Cav-KKSL to the droplets. Ca v-KKSL expression did not grossly alter cellular triacylglyceride or choles teryl levels, although droplet morphology was affected in some cells. These data suggest that accumulation of caveolin-1 to unusually high levels in t he ER causes targeting to lipid droplets. and that mechanisms must exist to ensure the rapid exit of newly synthesized caveolin-1 from the ER to avoid this fate.