Characterisation of two protein phosphatase 2A holoenzymes from maize seedlings

摘要

Two holoenzymes of protein phosphatase 2A (PP2A), designated PP2AI and PP2AII, were purified from maize seedlings. The subunit composition of maize holoenzymes generally resembled those of animal PP2A. Using SDS/PAGE and Western blots with antibodies generated against peptides derived from animal PP2A, we established the subunit composition of plant protein phosphatase 2A. In both maize holoenzymes, a 38 000 catalytic (PP2Ac) and a 66 000 constant regulatory subunit (A) constituting the core dimer of PP2A were present. In addition, PP2AI (180 000-200 000) contained a protein of 57 000 which reacted with antibodies generated against the peptide (EFDYLKSLEIEE) conserved in all eukaryotic Bα regulatory subunits. In contrast, none of the proteins visualised in PP2AII (14 000-160 000) by double staining reacted with these antibodies. The activity of PP2AI measured with ~(32)P-labelled phosphorylase a in the presence of protamine and ammonium sulfate is about two times higher than that of PP2AII. PP2AI and PP2AII displayed different patterns of activation by protamine, polylysine and histone H1 and exhibit high sensitivity toward inhibition by okadaic acid. The data obtained provide direct biochemical evidence for the existence in plants of PP2A holoenzymes composed of a catalytic subunit complexed with one or two regulatory subunits.