Involvement of Seipin in Directing Localized PA for Lipid Droplet Formation

Abstract

A loss of function of seipin, an endoplasmic reticulum (ER) membrane protein causes a severe congenital generalized lipodystrophy in humans. In yeast, deletion of seipin, FLD1, results in aberrant lipid droplets. However, the molecular mechanism of seipin is still unknown. Fei et al. (2011) reported that the deletion of FLD1 results in an increase in membrane phosphatidic acid (PA) concentration. In this study, we use Opi1p, a transcription repressor involved in phospholipid synthesis, as a PA sensor. Opi1p is known to bind to PA as well as the ER resident protein Scs2p. We report that Opi1p‐GFP forms puncta in the absence of FLD1. These puncta, usually next to droplets, are more pronounced with an Opi1p mutant (Opi1pFFAT) that cannot bind to Scs2p but still binds to PA. In order to test whether lipid droplets are necessary for the Opi1p‐GFP puncta in fld1Δ cells, we expressed Opi1p‐mCherry both in a mutant that cannot generate neutral lipids nor droplets (4KO; are1are2lro1dga1 KO), and one which also lacks FLD1 (5KO). Puncta were attenuated in these strains compared to fld1Δcells. In combination with the finding that Opi1p transiently localizes to droplets during inositol starvation (Chang et al, poster in this meeting), our data suggest that PA flux is important for droplet formation and that Fld1p facilitates utilization of PA during lipid droplet biogenesis. Supported by NIH grant GM084210.