Distinct conformational changes in the catalytic subunit of cAMP-dependent protein kinase around physiological conditions. Do these changes reflect an ability to assume different specificities?

Abstract

The free catalytic subunit of cAMP-dependent protein kinase readily undergoes a pronounced, salt-induced conformational change at neutral pH and around physiological values of ionic strength. This change, which is fully reversible, can be monitored directly by the relative chemical reactivity of two SH groups in the enzyme. Upon increasing the ionic strength of the medium from 0.03 to 0.22, one sulfhydryl becomes more reactive towards 5,5′-dithiobis[2-nitrobenzoic acid] while the other sulfhydryl becomes less reactive towards the same reagent. In parallel, the enzyme undergoes a salt-induced inactivation when histone H2b is used as a substrate. Though not reflected in the V max, this conformational change considerably increases the K m of the enzyme for histone H2b as well as for MgATP. This intrinsic malleability of the enzyme can account for the well-known salt inhibition of the enzyme for certain substrates and ion-dependent activation towards other substrates. It is suggested that this malleability might constitute the molecular basis for modulating the specificity of the enzyme and channeling its activity from one substrate to another in response to intracellular specifier signals.