Studies on Functions of the 63-kDa A- and 74-kDa B'.DELTA.-Regulatory Subunits in Human Erythrocyte Protein Phosphatase 2A: Dissociation and Reassociation of the Subunits.

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A heterodimeric form, CA, of protein-serine/threonine phosphatase (PP) 2A purified from human erythrocytes was dissociated into a 34-kDa catalytic subunit C and 63-kDa inactive subunit A by Sephacryl S-200 gel filtration in the presence of 6M urea. Reassociation of the C- and A-subunits in the absence of urea suppressed the PP activity of the C subunit toward phosphorylase α, P-H2B histone, and P-H1 histone in the presence or absence of 20mM MnCl2, or 50mM Mg(CH3COO)2but stimulated the PP activity toward P-H1 histone in the presence of 200mM NaCl and the Mn2+-dependent protein-tyrosine phosphatase (PTP) activity toward P-Tyr-Glu copolymers. The 74-kDa inactive B'a subunit was isolated from a heterotrimeric form, CAB'δ, of PP2A partially purified from human erythrocytes, by heparin-Sepharose column chromatography. The B'δ subunit reassociated with CA and suppressed the PP- and PTP-activities of CA. The B'δ subunit did not associate with the isolated C subunit directly, and had no effect on the activities of the C subunit, indicating that the A subunit is essential for the association of the B'δ subunit with CA and the resulting suppression of the PP- and PTP-activities.
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