Limited proteolytic digestion and dissociation of smooth muscle phosphatase-I modifies its substrate specificity. Preparation and properties of different forms of smooth muscle phosphatase-I.
Smooth muscle phosphatase-I (SMP-I), a protein phosphatase purified from turkey gizzard smooth muscle, is composed of 2 regulatory subunits (Mr = 60,000 and 55,000) and a catalytic subunit (Mr = 38,000). Two other forms of this enzyme have been prepared and characterized. The free catalytic subunit, termed SMP-Ic, was prepared by ethanol treatment of SMP-I, and a form devoid of the 55,000-Da subunit, termed SMP-I2, was prepared by limited tryptic digestion. Exposure of SMP-I to proteases like trypsin and chymotrypsin results in a rapid degradation of the 55,000-Da polypeptide. Degradation of the catalytic subunit is observed only upon prolonged digestion. The 60,000-Da polypeptide appears to be resistant to the action of trypsin and chymotrypsin. SMP-I dephosphorylates myosin light chains but is not active toward intact myosin or heavy meromyosin. However, when the catalytic subunit is dissociated from both regulatory subunits or from the 55,000-Da polypeptide, the enzyme becomes active toward myosin suggesting that the 55,000-Da polypeptide inhibits the activity of the catalytic subunit toward myosin. In addition to alteration of the substrate specificity, the regulatory subunits also modulate the effect of divalent cations, like Mn2+, on the activity of the enzyme.
D000818:Animals; D002918:Chymotrypsin / Q000494:pharmacology; D004591:Electrophoresis, Polyacrylamide Gel; D007700:Kinetics; D008345:Manganese / Q000494:pharmacology; D008970:Molecular Weight; D015879:Myosin Subfragments / Q000378:metabolism; D043263:Myosin-Light-Chain Phosphatase; D009218:Myosins / Q000378:metabolism; D010749:Phosphoprotein Phosphatases / Q000032:analysis*; D013379:Substrate Specificity; D014357:Trypsin / Q000494:pharmacology; D014422:Turkeys
D015879:Myosin Subfragments; D008345:Manganese; D010749:Phosphoprotein Phosphatases; D043263:Myosin-Light-Chain Phosphatase; D002918:Chymotrypsin; D014357:Trypsin; D009218:Myosins