Crystal Structure of Type IIa Holoenzyme of PKA Defines the Molecular Basis of Isoform Diversity

Abstract

The biological significance of the presence of two major isoforms (Type I and II) of the cAMP-dependent protein kinase (PKA) is still uncertain. The previously published Type Iα holoenzyme structure of PKA reveals an extended interface between the catalytic (C) and regulatory (RIα) subunits. To form the complex RIα undergoes major conformational changes whereas the C-subunit remains remarkably stable. Here we report the crystal structure of an RIIα holoenzyme of PKA which contains two cAMP-binding domains, referred to as domain A and B. This structure defines a very similar interface that consists of three components: the large lobe of the C-subunit, domain A as well as the common inhibitor site of R-subunit, and the helices A and B from domain B of the R-subunit. These are also similarities to RIα in the docking interface, suggesting that the two different isoforms of PKA are activated by cAMP through the same molecular mechanism. However, significant conformational differences can also be observed: the open state of C-subunit, the overall rotation of domain B, and the interactions between the linker and B/C helix on the C-subunit. These differences clearly demonstrate the molecular basis for isoform diversity. They also provide for a different allosteric network of communication between domain A and B and for communication with the catalytic subunit.