A specific substrate from rabbit cerebellum for guanosine 3':5'-monophosphate-dependent protein kinase. I. Purification and characterization.

A protein that exhibits greater substrate specificity for cGMP-dependent protein kinase than for cAMP-dependent protein kinase has been purified 8,000-fold from cytosol of rabbit cerebellum to apparent homogeneity as judged by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The protein, termed G-substrate, is a monomer of 23,000 daltons. It is heterogeneous on isoelectric focusing, exhibiting three isoelectric forms over the pH range of 5.2-5.6 cGMP-dependent protein kinase catalyzes the incorporation of 2 mol of phosphate/mol of G-substrate, both into threonine residues. The protein has a high content of aspartate, glutamate, and proline. The hydrodynamic properties, heat stability, and acid solubility of this protein are consistent with an unfolded, nonglobular structure. G-substrate is localized primarily in the cytosol of cerebellum, although low concentrations of a phosphorylated protein with a similar molecular weight are detected in other brain regions.