Phosphodiesterase 313 (PDE3B), a major PDE isoform in adipocytes, plays a pivotal role in the anti-lipolytic action of insulin. Insulin phosphorylates and activates PDE3B in a phosphatidylinositol 3-kinase-depen dent manner. We identified a new 50 kDa protein that is phosphorylated by insulin and is co-immunoprecipitated with PDE3B by anti-PDE3B antibodies in rat adipocytes. The insulin-induced phosphorylation of the 50 kDa protein was also detected in a cell free system against the N-terminal and the catalytic regions, which are more than 700 amino acids apart recognize the 50 kDa protein, suggesting that it is not a proteolytic product, but an associated protein with PDE313. Phosphoamino acid analysis indicated that both serine and threonine residues in the 50 kDa protein were phosphorylated, but only serine residues in PDE313 were phosphorylated. Therefore, it appears likely that this is a new protein which is associated with PDE3B. (c) 2005 Elsevier Inc. All rights reserved.