Purified rabbit liver glycogen synthase was found to be a substrate for six different protein kinases: (i) cyclic AMP-dependent protein kinase, (ii) two Ca2+-stimulated protein kinases, phosphorylase kinase (from muscle) and a calmodulin-dependent glycogen synthase kinase, and (iii) three members of a Ca2+ and cyclic nucleotide independent class, PC(0.7), F(A)/GSK-3, and casein kinase-1. Greatest inactivation accompanied phosphorylation by cyclic AMP-dependent protein kinase (to 0.5-0.7 phosphate/subunit, x̄glucose-6-P activity ratio reduced from approximately 1 to 0.6) or F(A)/GSK-3 (to ~1 phosphate/subunit, activity ratio, 0.46). Phosphorylation by the combination F(A)/GSK-3 plus PC(0.7) was synergistic, and more extensive inactivation was achieved. The phosphorylation reactions just described caused significant reduction in the V(max) of the glycogen synthase with little effect on the S(0.5) (substrate concentration corresponding to V(max)/2). Phosphorylase kinase achieved a lesser inactivation, to an activity ratio of 0.75 at 0.6 phosphate/subunit. PC(0.7) acting alone, casein kinase-1, and the calmodulin-dependent protein kinase did not cause inactivation of liver glycogen synthase with the conditions used. Analysis of CNBr fragments of phosphorylated glycogen synthase indicated that the phosphate was distributed primarily between two polypeptides, with apparent M(r) = 12,300 (CB-I) and 16,0000-17,0000 (CB-II). PC(0.7) and casein kinase-1 displayed a decided specificity for CB-II, and the calmodulin-dependent protein kinase was specific for CB-II. The other protein kinases were able, to some extent, to introduce phosphate into both CB-I and CB-II. Studies using limited proteolysis indicated that CB-II was located at a terminal region of the subunit. CB-I contains a minimum of one phosphorylation site and CB-II at least three sites. Liver glycogen synthase is therefore potentially subject to the same type of multisite regulation as skeletal muscle glycogen synthase although the muscle and liver enzymes display significant differences in both structural and kinetic properties.
|Original language||English (US)|
|Number of pages||8|
|Journal||Journal of Biological Chemistry|
|State||Published - Jan 1 1984|
ASJC Scopus subject areas
- Molecular Biology
- Cell Biology