PROGRESSIVE CYCLIC NUCLEOTIDE-INDUCED CONFORMATIONAL-CHANGES IN THE CGMP-DEPENDENT PROTEIN-KINASE STUDIED BY SMALL-ANGLE X-RAY-SCATTERING IN SOLUTION

Small angle scattering data from bovine lung type I alpha cGRSP-depend ent protein kinase (PKG) in the absence of cGMP show the protein to ha ve a highly asymmetric structure with a radius of gyration (R-g) of 45 Angstrom and a maximum linear dimension (d(max)) of 165 Angstrom. The addition of cGMP induces a marked conformational change in PKG. The R -g and d(max) increase 25-30%, and the protein's mass moves further aw ay from the center of mass; this results in an even more asymmetric st ructure, Fourier transform infrared spectroscopy data suggest that the conformational change induced by cGMP binding is primarily due to a t opographical movement of the structural domains of PKG rather than to secondary structural changes within one or more of the individual doma ins. Each monomer of the dimeric PKG contains one high and one low aff inity cGMP-binding site. A prominent increase in the asymmetry of PKG occurs with binding to high affinity cGMP-binding sites alone, but the full domain movements require the binding to both sets of sites, Thes e conformational changes occurring in PKG with the progressive binding of cGMP to both sets of cGMP-binding sites correlate with past data, which have indicated that cGMP binding to both sets of sites is requir ed for the fall activation of the enzyme, These results provide the fi rst quantitative measurement of the overall PKG structure, as well as an assessment of the structural events that accompany the activation o f a protein kinase upon binding a small molecular weight ligand.