Properties of Phosphatases Isolated from Smooth Muscles

Properties of Phosphatases Isolated from Smooth Muscles 

1985) proved to be a trimeric form of PP2A, and this enzyme dephosphorylated intact myosin and affected smooth muscle contractility in skinned fiber preparations (Bialogjan et al., 1987), suggesting that certain types of trimeric PP2A might be involved in the de-phosphorylation of myosin in vivo. 

Pato and coworkers (Pato and Adelstein, 1983 Pato and Kerc, 1985, 1986) separated and purified four smooth muscle protein phosphatases, identified as SMP-I, SMP-II, SMP-III, and SMP-IV. SMP-I and SMP-II dephosphorylated isolated LC20, and these two enzymes were classified as PP2A and PP2C, respectively (Pato and Adelstein, 1983). SMP-III and SMP-IV dephosphorylated both isolated LC20 and intact myosin, and these enzymes were purified and characterized (Pato and Kerc, 1985 Tulloch and Pato, 1991). SMP-IV was composed of 40- and 58-kDa subunits and eluted 

From this summary it is clear that several phosphatases may dephosphorylate myosin and there is no obvious way to identify the in vivo activity. One approach was to characterize the phosphatase that is associated with myosin (or actomyosin) on the assumption that binding to myosin is a property expected for MLCP. Using okadaic acid as a probe it was found that the major phosphatase in gizzard actomyosin preparations was PPl (Ishihara et al, 1989). Subsequently several groups isolated MLCP using myosin or actomyosin as a source. The preparation of Mitsui et al. 

A myofibrillar form of MLCP distinct from those just described was isolated from turkey gizzards by Nowak et al. (1993). This consisted of a monomeric 35-kDa component that bound to unphosphorylated myosin. It was suggested that in vivo this protein is complexed with MLCK and a 63-kDa subunit. The relationship of this MLCP to the trimeric phosphatases is not known. In contrast, several other groups found that PPlc did not bind to unphosphorylated myosin (Alessi et al., 1992 Mitsui et al., 1992 Okubo et al., 

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B. Properties of Phosphatases Isolated from Smooth Muscles... 

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