Purine nucleotide- and sugar phosphate-induced inhibition of the carboxyl methylation and catalysis of protein phosphatase-2A in insulin-secreting cells: Protection by divalent cations

Recently, we demonstrated that the 36 kDa catalytic subunit of protein phos phatase 2A (PP2Ac) undergoes methylation at its C-terminal leucine in norma l rat islets, human islets and isolated beta cells; this modification incre ases the catalytic activity of PP2A [Kowluru el al. Endocrinology. 137:2315 -2323, 1996]. Previous studies have suggested that adenine and guanine nucl eotides or glycolytic intermediates [which are critical mediators in beta c ell function] also modulate phosphatase activity in the pancreatic beta cel l. Therefore, we examined whether these phosphorylated molecules specifical ly regulate the carboxyl methylation and the catalytic activity of PP2A in beta cells. Micromolar concentrations of ATP, ADP, GTP or GDP each inhibite d the carboxyl methylation of PP2Ac and, to a lesser degree, the catalytic activity of PP2A. Likewise, the carboxyl methylation of PP2Ac and its catal ytic activity were inhibited by [mono- or di-] phosphates of glucose or fru ctose. Additionally, however, the carboxyl methylation of PP2Ac was signifi cantly stimulated by divalent metal ions (Mn2+ > Mg2+ > Ca2+ > control). Th e nucleotide or sugar phosphate-mediated inhibition of carboxyl methylation of PP2Ac and the catalytic activity of PP2A were completely prevented by M n2+ or Mg2+. These data indicate that divalent metal ions protect against t he inhibition by purine nucleotides or sugar phosphates of the carboxyl met hylation of PP2Ac perhaps permitting PP2A to function under physiologic con ditions. Therefore, these data warrant caution in interpretation of extant data on the regulation of phosphatase function by purine nucleotides.