Adipose tissue protein phosphatase inhibitor‐2

Abstract

Rat fat cells contain three species of spontaneously active inhibitor proteins of protein phosphatase 1, as resolved by SDS‐PAGE, with apparent molecular masses of 40 kDa, 33 kDa and 28 kDa respectively. The 33‐kDa, thermostable inhibitor was highly purified from bovine adipose tissue and shown to be very similar to inhibitor‐2 of skeletal muscle. It was phosphorylated, on threonine only, by glycogen synthase kinase 3. It formed an inactivated complex with protein phosphatase 1, that was reactivated by incubation with ATP‐Mg and glycogen synthase kinase 3. By gel filtration it had a Stokes radius of 3.4 nm. Peptide and phosphopeptide maps, generated by Staphylococcus aureus V8 proteinase, trypsin or thermolysin, of the inhibitor and of the skeletal muscle inhibitor‐2 were similar. The 40‐kDa inhibitor, which was denatured by boiling, represents a novel protein phosphatase inhibitor protein or an undegraded precursor of inhibitor‐2. The total activity of inhibitor‐2‐like material (thermostable and macromolecular) in an adipocyte cytosol extract corresponded to an intracellular concentration of 0.3 μM inhibitor‐2. Copyright © 1988, Wiley Blackwell. All rights reserved