Evidence for a "mute" catalytic subunit of cyclic AMP-dependent protein kinase from rat muscle and its mode of activation.

An isoenzyme of the catalytic subunit of type II cyclic AMP-dependent protein kinase from rat muscle is reported which coelutes with the classical catalytic subunit but differs from it in isoelectric point (pI 8.7 vs pI 9.1) and is enzymmatically inactive. After reaction with a heat- and acid-stable component of the protein kinase modulator fraction from the same tissue, the "mute" isoenzyme displays a high activity when assayed on isoelectric focusing gels. This activation process does not occur through proteolytic degradation and is not characteristic of a turnover-type reaction. The data imply direct interaction between the isoenzyme and a modulating protein which may subsequently be separated from the enzyme without reversal of the activation. The modulator protein thus appears to act as a template, inducing a conformational change. The implications of such a mute isoenzyme and its control through small modulator proteins are discussed.