Identification by amino acid sequencing of three major regulatory phosphorylation sites on rat acetyl‐CoA carboxylase

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Abstract
We have examined the sites phosphorylated on acetyl‐CoA carboxylase by three protein kinases which have been shown to inactivate the enzyme, i.e. cyclic‐AMP‐dependent protein kinase, acetyl‐CoA carboxylase kinase‐2 (ACK2, purified from rat mammary gland) and the AMP‐activated protein kinase (formerly called acetyl‐CoA carboxylase kinase‐3, purified from rat liver). Each protein kinase phosphorylates two out of three sites (termed 1–3) which have been established by amino acid sequencing. The two sites phosphorylated by each kinase can be recovered on separate peptides, TC1 and TC2, derived by combined digestion of the native enzyme by trypsin and chymotrypsin: TC1 = Ser‐2Ser(P)‐Met‐3Ser(P)‐Gly‐Leu; TC2 = Arg‐Met‐1Ser(P)‐Phe‐Cyclic‐AMP‐dependent protein kinase phosphorylates sites 1 and 2 exclusively, whereas the AMP‐activated protein kinase phosphorylates sites 1 and 3, plus at least one other minor site. ACK2 phosphorylates site 1 and, more slowly, an unidentified site(s) within TC1.We have also established the structures of the single major phosphopeptides (T1 and C1 respectively) which are recovered by HPLC after acetyl‐CoA carboxylase phosphorylated by cyclic‐AMP‐dependent protein kinase is digested with trypsin or chymotrypsin alone. T1 is related to TC1, and has the structure: Ser‐Ser(P)‐Met‐Ser‐Gly‐Leu‐His‐Leu‐Val‐Lys. C1 is identical with TC2.We have carried out studies on the correlation of the activity of acetyl‐CoA carboxylase with the occupancy of sites 1, 2 and 3 during phosphorylation by each of the three protein kinases. The results suggest that phosphorylation of site 3 is primarily responsible for the large decrease in Vmax produced by the AMP‐activated protein kinase, while phosphorylation of site 1 may be primarily responsible for the increase in A0.5 for citrate and more modest depression of Vmax produced by cyclic‐AMP‐dependent protein kinase and ACK2.Our results emphasize that amino acid sequence information is essential in the unequivocal interpretation of data from phosphopeptide mapping experiments and allow a more complete interpretation of previous data on phosphorylation of acetyl‐CoA carboxylase in intact cells. They also open the way to experiments which could establish the physiological roles of these protein kinases in the control of fatty acid synthesis.

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