Partial purification and characterization of sodium channel phosphatases from rat brain: Similarity to phosphatase 2A

Abstract

Four distinct serine/threonine protein phosphatases have been purified from and identified in various tissues. They are type 1 and type 2 phosphatases, which are further classified as phosphatase 2A, 2B, and 2C. In this study, endogenous brain phosphatases that dephosphorylate sodium channels were partially purified and characterized. Multiple peaks of sodium channel phosphatase were detected after DEAE-Sephadex chromatography and gel filtration. All peaks were sensitive to a low concentration of okadaic acid (10 nM), which strongly suggests that phosphatase 2A is the major brain phosphatase dephosphorylating sodium channels. Individual fractions containing sodium channel phosphatase activity from both DEAE-Sephadex chromatography and gel filtration were subjected to immunoblot with anti-phosphatase 2Ac antibody. The results indicate that all fractions containing phosphatase activity also contained phosphatase 2Ac immunoreactivity. The fractions which stained most intensely in the immunoblots were the fractions containing the highest phosphatase activity in all cases. The molecular weights of the multiple sodium channel phosphatases estimated by gel filtration were 83 kDa, 147 kDa, and 141 kDa. These may represent isozymes of phosphatase 2A in brain.