A LATENT FORM OF PROTEIN PHOSPHATASE 1-ALPHA ASSOCIATED WITH BOVINE HEART MYOFIBRILS

The catalytic subunit of the major protein phosphatase associated with bovine cardiac myofibrils was purified to homogeneity. Sodium dodecyl sulfate polyacrylamide gel electrophoresis of the enzyme revealed onl y one band with an apparent molecular weight of 37 000. On gel filtrat ion chromatography, the phosphatase activity and the protein co-eluted as a single peak with an apparent molecular weight of 37 000. The pur ified enzyme was identified as the catalytic subunit of protein phosph atase 1, as determined by sensitivity to inhibitor 1, inhibitor 2, oka daic acid and by specific immunostaining. Evidence obtained with speci fic antipeptide antibodies demonstrated that this myofibril protein ph osphatase was predominately the alpha isoform of protein phosphatase 1 . The purified catalytic subunit was completely inactive. It was activ ated by pretreatment with Co2+/trypsin in the presence of high ionic s trength. Treatment with trypsin alone did not activate the latent enzy me. The enzyme was also activated by Co2+ or Mn2+ alone but not by Ca2 + Mg2+, Ni2+, Cu2+ or Zn2+ Activation of the enzyme was not reversed b y removal of Co2+, but Mn2+-activated phosphatase activity was partial ly reversed when Mn2+ was removed. The catalytic subunit could form a 1:1 complex with inhibitor 2 in vitro. The resulting holoenzyme was al so activated by pretreatment with Co2+. Since phosphatase 1 alpha is t he major phosphatase associated with cardiac myofibril, it is suggeste d that it is responsible for the dephosphorylation of myosin and other myofibril phosphoproteins.