PHOTOAFFINITY-LABELING OF CYCLIC GMP-BINDING PROTEINS IN HUMAN PLATELETS

The photoaffinity labelling of platelet cyclic GMP (cGMP)-binding prot eins by [P-32]cGMP was studied; at least five labelled proteins (110, 80, 55, 49 and 38 kDa) were detected in platelet cytosol and four (80, 65, 49 and 38 kDa) in platelet membranes. The 110 kDa species was ide ntified as cGMP-inhibited cyclic AMP (cAMP) phosphodiesterase (PDE III ) by immuno-precipitation and by the inhibition of photolabelling by s pecific inhibitors of this enzyme. Similarly, the 80 kDa species was i dentified as cGMP-dependent protein kinase by immuno-precipitation and by the effects of cGMP analogues on photo-labelling. Addition of cAMP greatly enhanced the labelling of this 80 kDa protein, implying the e xistence of a potentially important interaction between the effects of cGMP and cAMP. The 65 kDa photolabelled protein appears to be a novel platelet cyclic-nucleotide-binding protein. In contrast, the 49 and 5 5 kDa photolabelled species are probably the RI and RII regulatory sub units of cAMP-dependent protein kinase, and the 38 kDa protein(s) may be proteolytic fragment(s) of RI and/or RII.