C Subunits binding to the protein kinase a RIα dimer induce a large conformational change

We present structural data on the RIalpha isoform of the cAMP-dependent protein kinase A that reveal, for the first time, a large scale conformational change within the RIalpha homodimer upon catalytic subunit binding. This result infers that the inhibition of catalytic subunit activity is not the result of a simple docking process but rather is a multi-step process involving local conformational changes both in the cAMP-binding domains as well as in the linker region of the regulatory subunit that impact the global structure of the regulatory homodimer. The results were obtained using small-angle neutron scattering with contrast variation and deuterium labeling. From these experiments we derived information on the shapes and dispositions of the catalytic subunits and regulatory homodimer within a holoenzyme reconstituted with a deuterated regulatory subunit. The scattering data also show that, despite extensive sequence homology between the isoforms, the overall structure of the type Ialpha holoenzyme is significantly more compact than the type IIalpha isoform. We present a model of the type Ialpha holoenzyme, built using available high-resolution structures of the component subunits and domains, which best fits the neutron-scattering data. In this model, the type Ialpha holoenzyme forms a flattened V shape with the RIalpha dimerization domain at the point of the V and the cAMP-binding domains of the RIalpha subunits with their bound catalytic subunits at the ends.