Possibility of shape conformers of the protein inhibitor of the cyclic adenosine monophosphate-dependent protein kinase

Abstract

The heat-stable, protein inhibitor of the cyclic AMP dependent protein kinase was purified to homogeneity from rabbit skeletal muscle by preparative electrophoresis. Multiple charged forms of the inhibitor on diethylaminoethyl chromatography were detected; the form that was further characterized is the predominant species in skeletal muscle comprising greater than 70% of the total. The apparent MW 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 MW of 11,000. These 2 forms are designated I and I'', respectively. The I form migrates with an apparent MW of 10,000 on nondenaturing gel electrophoresis and of 10,500-11,500 on sodium dodecyl sulfate (NaDodSO4) gel electrophoresis; the I'' form migrates with an apparent MW of 6500-8300 on NaDodSO4 electrophoresis and has a minimum MW of 10,400 by amino acid analysis. Taking into account the anomalous behavior displayed by low MW proteins with the various techniques employed, the I and I forms of the protein inhibitor may represent shape conformers.