Rat perilipin, the major protein at the surfaces of cytosolic lipid particles in adipocytes and steroidogenic cells (Blanchette-Mackie et al. 1995), is phosphorylated by protein kinase A catalytic subunit (Greenberg et al. 1991) on serine residues 81, 223, and 277 (Tansey et al. 2003). All three serine residues and the adjoining sequences that mediate phosphorylation (Cohen 1988) are conserved in mouse perilipin, while only the first and third are conserved in human perilipin. By inference, PKA targets these three mouse and two human serines as well. Phosphorylated perilipin is redistributed on the droplet surfaces (Souza et al. 1998). While two isoforms of rat perilipin protein are found on lipid particles in adipocytes, only the larger isoform appears to regulate lipolysis (Tansey et al 2003). The single human and mouse isoforms of perilipin correspond to the large rat isoform. In mouse 3T3-L1 cells, perilipin phosphorylation requires the presence of caveolin-1 at the surface of the lipid particle (Cohen et al. 2004). This positive regulatory effect of caveolin-1 is inferred for rat and human.