Abstract
A photoaffinity labeling method was used to characterize and compare cyclic nucleotide-binding proteins of bovine liver cytosol with binding proteins of the nucleus. After photoaffinity labeling of cytosol with 8-azido cyclic [(32)P]AMP, autoradiographs of sodium dodecyl sulfate polyacrylamide gel electrophoresis revealed two major labeled proteins of 47,000 and 52,000-55,000 daltons. DEAE-cellulose column-derived fractions suggested that the larger protein was the regulatory subunit of peak II cyclic AMP-dependent protein kinase and the smaller protein was the regulatory subunit of peak I kinase. The smaller protein was largely present as the free regulatory subunit. The two binding proteins differed in their ability to bind cyclic GMP. Binding to both proteins was abolished by excess unlabeled cyclic AMP but not by 5'-AMP. Photoaffinity labeling of a 0.14 M salt extract of nuclei and a nonhistone chromosomal protein preparation revealed two major binding proteins with the same molecular weight and competition profiles as those of the cytosol. Detergent-washed nuclei gave similar results. Several minor binding proteins were observed in both cytosol and nucleus. One protein (36,000 daltons) was unique to the nucleus and had low affinity for 8-azido cyclic AMP. Photoaffinity labeling with cyclic [(3)H]GMP revealed a cytosol protein, absent from the nucleus, of 31,000 daltons and the ligand was competed for by both cyclic GMP and 5'-GMP. These studies suggest that the major specific cyclic AMP-binding proteins of bovine liver are the type I and type II regulatory subunits of cyclic AMP-dependent protein kinase and are present in both nucleus and cytoplasm.