Identification and differential expression of two forms of regulatory subunits (RII) of cAMP-dependent protein kinase II in Friend erythroleukemic cells. Differentiation and 8-bromo-cAMP elicit a large and selective increase in the rate of biosynthesis of only one type of RII

The concentration of regulatory subunits (R) of type II cAMP-dependent protein kinase increased 4- to 5-fold when Friend erythroleukemic cells were either grown in medium containing 0.5 mM 8-bromo-cAMP and 0.2 mM methylisobutylxanthine or stimulated to differentiate. Two species of RII with apparent M(r) values of 54,000 (RII-54) and 52,000 (RII-52) are expressed in Friend cells. Both forms of RII were (a) covalently labeled with 8-N₃-[³²P]cAMP, (b) phosphorylated by the catalytic subunit of protein kinase II, and (c) complexed by polyclonal anti-RII IgGs. RII-52 and RII-54 were not interconverted by phosphorylation or dephosphorylation. A monoclonal antibody that recognizes an internal site in RII resolved the two cAMP-binding proteins by preferentially binding RII-54. The structural diversity suggested by the monoclonal antibody experiment was further examined by comparing two-dimensional maps of tryptic peptides obtained from metabolically labeled ([³⁵S]met) RII-52 and RII-54. Groups of ³⁵S-labeled peptides that were either uniquely derived from RII-54 or obtained only from RII-52 were readily distinguished, thereby demonstrating that Friend cells produce two separate and distinct forms of type II cAMP-binding subunits. The relative rate of synthesis of RII-52 increased 12- to 14-fold during erythroid differentiation and treatment with 8-bromo-cAMP, while the rate of RII-54 synthesis either declined slowly or was unchanged. Thus, two homologous forms of RII are subject to different modes of physiological (differentiation) and pharmacological (chronic 8-Br-cAMP) regulation, and the accumulation of total RII observed in the present and previous (Schwartz, D.A., and Rubin, C.S. (1983) J. Biol. Chem. 258, 777-784) studies results from a selective increase in the rate of biosynthesis of RII-52.