Human adipocytes express high levels of two distinct lipid droplet proteins, fat 
specific protein 27 (FSP27; also called CIDEC), a member of the CIDE family, and 
perilipin1 (PLIN1), a member of the PAT family. Both proteins play a role in fat 
metabolism in adipocytes, but how they interact is not known. Our present study 
demonstrates that FSP27 and PLIN1 co-localize and interact in cultured human 
primary adipocytes. We also found that the C-terminal domain of FSP27, aa 
120-220, interacts with PLIN1. Individual expression of exogenous FSP27 or PLIN1 
increased triglyceride content and decreased glycerol release (a measure of 
lipolysis), but co-expression of both proteins did not further increase 
triglyceride content or decrease lipolysis in human adipocytes. However, the 
combination of PLIN1 and FSP27 increased the average size of lipid droplets or 
caused the formation of unilocular adipocytes. Our data suggest that FSP27 
interacts with PLIN1 to regulate lipid droplet size in human adipocytes in a 
concerted manner.