Phosphorylation of smooth muscle myosin at two distinct sites by XXXX .

@entity73059 @entity233 @entity8723 @entity563 @entity1889 @entity4338
The 20,000-dalton light chain of @entity233 gizzard myosin is phosphorylated at two sites. Dual phosphorylation is observed when both intact myosin and isolated light chains are used as substrates. Phosphorylation of the second site is not observed at higher ionic strength (e.g. 0.35 M @entity4338 ). The first phosphorylation site ( @entity563 19) is phosphorylated preferentially to the second site. The latter is phosphorylated more slowly than the first site, and its phosphorylation requires relatively high concentrations of @entity73059 . It is suggested that @entity73059 catalyzes the phosphorylation of both sites on the light chain, and several reasons are cited that make it unlikely that a contaminant kinase is involved. The second phosphorylation site is a @entity1889 residue. Based on the results of limited proteolysis of the light chain, it is concluded that the @entity1889 residue is close to @entity563 19, and possible locations are threonines 9, 10, and 18. At all concentrations of @entity8723 , phosphorylation of the second site markedly increases the actin-activated ATPase activity of myosin and accelerates the superprecipitation response of myosin plus actin.
