Purification of a physiological form of the inhibitor protein of the cAMP-dependent protein kinase.

The inhibitor protein of the cAMP-dependent protein kinase exists in two forms which can be distinguished by separation techniques that characterize proteins on the basis of size (McPherson, J. M., Whitehouse, S., and Walsh, D. A. (1979) Biochemistry 18, 4835-4845). To date, only one of these forms has been purified and the form obtained was not the species that is present in the initial extracts of tissues. This paper reports the purification to homogeneity of the form of the inhibitor that is present in initial cell extracts and which, in consequence, appears to be a physiological size species of this protein.