PURIFICATION AND CHARACTERIZATION OF THE MAMMALIAN MYOSIN LIGHT-CHAINPHOSPHATASE HOLOENZYME - THE DIFFERENTIAL-EFFECTS OF THE HOLOENZYME AND ITS SUBUNITS ON SMOOTH-MUSCLE

We have purified to homogeneity from the myofibrillar fraction of pig bladder a mammalian heterotrimeric form of PP-1, SMPP-1M. Purified pig bladder SMPP-1M is similar in composition and substrate specificity t o avian gizzard PP-1M reported by Alessi et al. (Alessi, D., Macdougal l, L. K., Sola, M. M., Ikebe, M., and Cohen. P. (1992) Eur. J. Biochem . 210, 1023-1035) and consists of the catalytic subunit of PP-1 (37 kD a) and two other equimolar subunits of 130 and 20 kDa. The properties of SMPP-1M and the role of its regulatory subunits in the dephosphoryl ation of myosin and in the initiation of relaxation were characterized both in vitro and in smooth muscle, We show that the relaxant effect of the catalytic subunit in smooth muscle is markedly potentiated by t he addition of the regulatory subunits of SMPP-1M, Our findings demons trate that SMPP-1M is the major phosphatase dephosphorylating myosin i n mammalian smooth muscle and that myosin dephosphorylation is regulat ed in. vivo via targeting subunits that specifically alter the substra te specificity of PP-1C toward myosin.