BOLE OF PROTEIN TARGETING TO GLYCOGEN (PTG) IN THE REGULATION OF PROTEIN PHOSPHATASE-1 ACTIVITY

We have recently cloned from 3T3-L1 adipocytes a novel glycogen-tageti ng subunit of protein phosphatase-1, termed PTG (Printen, 5, A., Brady , M. J., and Saltiel, A. R. (1997) Science 275, 1475-1478), Differenti ation of 3T3-L1 fibroblasts into highly insulin-responsive adipocytes resulted in a marked increase: in PTG expression, Immobilized glutathi one S-transferase (GST)-PTG fusion protein specifically bound either P P1 or phosphorylase a, Addition of soluble GST-PTG to 3T3-L1. lysates increased PP1 activity against P-32-labeled phosphorylase a by decreas ing the K-m of PP1 for phosphorylase 5-fold, while having ma effect on the V-max, of the dephosphorylation reaction, Alternatively, PTG did not affect PP1 activity against hormone-sensitive lipase, PTG was not a direct target of intracellular signaling, as insulin or forskolin tr eatment of cells did not activate a kinase capable of phosphorylating PTG in vivo or irt vitro, Finally, PTG decreased the ability of DARPP- 32 to inhibit PP1 activity from 3T3-L1 adipocyte lysates, These data c umulatively suggest that PTG increases PPI activity against specific p roteins by several distinct mechanisms.