Heat-stable regulatory factors are associated with polycation-modulable phosphatases

Several protein phosphatases which we designated PCM-I, PCM-II and PCM-IId were identified in preparations from aortic smooth muscle. A unique feature of these enzymes is that phosphatase activities expressed against different substrates are subject to modulation by polycationic effectors such as polylysine and lysine-rich histone-H1. They can be distinguished from each other by virtue of significant quantitative differences regarding (a) relative substrate specificities exhibited against phosphorylated myosin light chains, phosphorylase, and inhibitor-1, (b) apparent molecular weights as determined by sucrose density centrifugation, and (c) differential susceptibility to polylysine-mediated modulation of phosphatase activities. Surprisingly, gel filtration of the very same PCM-phosphatase preparation yields either of two apparently different enzymes: namely PCM-II, or PCM-IId. The enzymes appear similar in that low concentrations of polylysine (0.03-0.13 microM) inhibit dephosphorylation of light chains by either enzyme and high concentrations inhibit dephosphorylation of phosphorylase a. However, PCM-II exhibits higher basal phosphatase activity against myosin light chains (480 U/mg) than against phosphorylase a (175 U/mg). In contrast, PCM-IId is more effective in dephosphorylating phosphorylase a (180 U/mg) than in dephosphorylating the light chains (88 U/mg). Moreover, phosphorylase phosphatase activity of PCM-II is biphasically stimulable by low concentrations of polylysine (0.01-0.5 microM), but no stimulation is seen with PCM-IId. In addition, earlier studies with PCM-I showed that, unlike either PCM-II or PCM-IId polylysine biphasically stimulated the dephosphorylation of both phosphorylase a and the myosin light chains. Nevertheless, in spite of these obvious differences between the enzymes other data suggests that the PCM-phosphatases may be related to each other. Incubation of PCM-II at 90 degrees C for 10 min apparently releases heat-stable regulatory proteins which reverse the modulatory effects of polylysine on light chain and phosphorylase phosphatase activities expressed by either PCM-II or PCM-I. Similarly, heat-stable proteins released from PCM-I also reverse polylysine-mediated modulation of both PCM-I and PCM-II. These findings are consistent with a working hypothesis suggesting that PCM-phosphatase may consist of a modulatory domain containing several regulatory proteins and a catalytic domain.(ABSTRACT TRUNCATED AT 400 WORDS)