Proteins of similar to 35, 55 and 65 kDa were purified from cauliflower extracts by microcystin-Sepharose chromatography and identified by amino acid sequencing as plant forms of protein (serine/threonine) phosphatase 1 (PP1) catalytic subunit, PP5 and a regulatory A-subunit of PP2A, respectively. Peptides that corresponded both to the tetratricopeptide (TPR) repeat and catalytic domains of PP5 were identified. Similar to mammalian PP5, the casein phosphatase activity of plant PP5 was activated > 10-fold by arachidonic acid, with half-maximal stimulation occurring at similar to 100 mu M lipid (C) 1999 Federation of European Biochemical Societies.