Properties of a Phosphoprotein Phosphatase from Skeletal Muscle and Its Regulation in Diabetes 1

Although the major phosphorylase phosphatase activity in skeletal muscle (1) and in liver (2) chromatographs with an apparent molecular weight of 250,000–260,000, only low molecular weight forms (30,000 to 35,000) have been purified to homogeneity (see review (3)). Since the low molecular weight phosphatases have not been demonstrated in tissues, it is important to isolate native high molecular weight phosphatases to investigate their subunit structure, substrate specificity, and mode of regulation.

In skeletal muscle partially purified phosphatase preparations of molecular weight ranging from 30,000 to 140,000 have been reported (3–7) whereas in other tissues investigators have obtained highly purified high molecular weight multisubunit phosphoprotein phosphatases. Tsuiki and co-workers (8, 9) have isolated two high molecular weight phosphatases from rat liver (termed phosphatase IB and II). Phosphatase IB has an apparent molecular weight of 260,000 and consists of three polypeptide chains of 69,000, 58,000, and 35,000 (9). Phosphatase II is of lower molecular weight (160,000) and comprises two subunits of 69,000 and 35,000 (8). A Ca²⁺-calmodulin-dependent protein phosphatase (termed phosphatase 2B) is composed of two polypeptide chains of 61,000 and 15,000 Da and is identical to a major calcium binding protein of neural tissue termed calcineurin (10). Two distinct protein phosphatases from turkey gizzard smooth muscle have been purified to apparent homogeneity (termed smooth muscle phosphatase I and II). Smooth muscle phosphatase I is composed of three polypeptide chains of 60,000, 55,000, and 38,000 Da (11) whereas the smooth muscle phosphatase II is composed of a single polypeptide chain of 43,000 Da (12). Furthermore, a protein phosphatase capable of catalyzing the dephosphorylation of the α subunit of eIf-2 has been shown to be composed of two polypeptide chains of 60,000 and 39,000 Da (13).