Role of the 78-kDa glucose-regulated protein as an activity modulator of protein phosphatase1 γ2

We have previously found the 78-kDa glucose-regulated protein (Grp78) to be a subunit of protein phosphatase1(PP1)gamma 2. To determine the role of Grp78 in PP1 gamma 2 holoenzyme, we compared the two forms of this enzyme, PP1 gamma 2 holoenzyme containing Grp78 and Grp78-dissociated PP1 gamma 2 in rat testes in terms of their kinetic constants and sensitivities to inhibitors of this enzyme. The enzymatic activity of the Grp78-dissociated enzyme was much lower at whole range of concentrations of a substrate (phosphorylase a) than that of the holoenzyme; the K-m value was about ten-fold higher in Grp78-dissociated enzyme than in holoenzyme, while the V-max was similar. IC(50)s of the Grp78-dissociated enzyme for three inhibitors (microcystin-LR, inhibitor-2, and okadaic acid) were more than ten-fold higher than those of the holoenzyme. These results indicate that the Grp78 subunit modulates the activity of PP1 gamma 2 through its actions to control the binding of substrates or inhibitors to PP1 gamma 2. (C) 1999 Academic Press.