Nuclear localization of an O-glycosylated protein phosphotyrosine phosphatase from human cells

Histochemical staining of immunoprecipitates of p65, a component of human M phase-promoting factor, identified the molecule as having phosphatase activity. The enzyme, purified 3400-fold from mitotic cell extracts by (NH4)2SO4 precipitation, DEAE chromatography, and immunoaffinity chromatography on immobilized anti-p65 IgG, was inhibited by Zn2- and Na3VO4 but not NaF or beta-glycerophosphate; 32P-labeled poly(Glu, Tyr) was more efficiently dephosphorylated than phosphorylated histone or phosphorylase a. Indirect immunofluorescence showed most of the phosphatase to be localized in the nucleus of interphase cells, with a fine, granular distribution unaltered by detergent extraction; in mitotic cells, p65 was localized on chromosomes. ELISA of subcellular fractions confirmed this localization. Immunoreactive p65 was recovered from immobilized wheat germ agglutinin (WGA) upon elution with N-acetylglucosamine; similarly, WGA recognized immunoaffinity-purified p65 on blots. Alkaline hydrolysis of blotted protein prevented WGA binding, indicating that phosphatase p65, like a small group of other nuclear proteins, contains O-linked carbohydrate terminating in N-acetylglucosamine.