ATP-citrate lyase in vivo contains three phosphorylation sites on two tryptic peptides (peptides A and B). These phosphorylation sites are under hormonal control. Multifunctional protein kinase (MFPK) from rat liver phosphorylates peptide B on serine and threonine residues whereas cAMP-dependent protein kinase phosphorylates peptide A on a serine residue (Ramakrishna, S., and Benjamin, W. B. (1985) J. Biol. Chem. 260, 12280-12286). We now report that rat adipose tissue MFPK also phosphorylates serine and threonine residues of peptide B of ATP-citrate lyase. When the activity of MFPK was assayed using partially purified (by chromatography on phosphocellulose) cytosol fractions from insulin-treated adipose tissue, it was found that MFPK activity was decreased by over 55%. This decrease in MFPK activity occurs at physiological concentrations of insulin (EC50 = 1 x 10(-10) M). Its onset is rapid and almost maximal at 5 min after the addition of insulin. Even when new protein synthesis is inhibited by cycloheximide, extracts from insulin-treated fat pads have less MFPK activity compared to the control. The insulin effect is maintained after further chromatography on a gel filtration column suggesting that the decrease in MFPK activity is not due to a low molecular weight inhibitor. The insulin-induced decrease in MFPK activity is due to a decrease in Vmax whereas the affinity of this enzyme toward ATP-citrate lyase or ATP is unchanged.