POSSIBILITY OF SHAPE CONFORMERS OF THE PROTEIN INHIBITOR OF THE CYCLIC ADENOSINE-MONOPHOSPHATE DEPENDENT PROTEIN-KINASE

The heat-stable, protein inhibitor of the cyclic adenosine monophosphate (cAMP) dependent protein kinase [Walsh, D. A., Ashby, C. D., Gonzalez, C., Calkins, D., Fischer, E., & Krebs, E. (1971a) J. Biol. Chem. 246, 1977—1985] has been purified to homogeneity from rabbit skeletal muscle by preparative electrophoresis. Employing a more sensitive assay system, we detected multiple charged forms of the inhibitor on diethylaminoethyl chromatography; the form that has been further characterizd is the predominant species in skeletal muscle comprising greater than 70% of the total. The apparent molecular weight of the protein inhibitor, as determined by Sephadex G-75 gel exclusion chromatography, is 22 000 in initial cellular extracts and at all stages during the purification prior to the final purification step of preparative gel electrophoresis, after which the homogeneous protein exhibits a molecular weight of 11000. These two forms are designated I and I', respectively. The I form migrates with an apparent molecular weight of 10000 on nondenaturing gel electrophoresis and of 10500-11500 on sodium dodecyl sulfate (NaDodSO4) gel electrophoresis; the I' form migrates with an apparent molecular weight of 6500-8300 on NaDodSO4 electrophoresis and has a minimum molecular weight of 10400 by amino acid analysis. Taking into account the anomalous behavior displayed by low molecular weight proteins with the various techniques employed, we suggest that the I and I' forms of the protein inhibitor may represent shape conformers. © 1979, American Chemical Society. All rights reserved.