The structures of protein kinase A in complex with CFTR: mechanisms of phosphorylation and reversible activation

Protein kinase A (PKA) is a key regulator of cellular functions by selectively phosphorylating numerous substrates, including ion channels, enzymes, and transcription factors. It has long served as a model system for understanding the eukaryotic kinases. Using cryo-electron microscopy, we present complex structures of the PKA catalytic subunit (PKA-C) bound to a full-length protein substrate, the cystic fibrosis transmembrane conductance regulator (CFTR) – an ion channel vital to human health. CFTR gating requires phosphorylation of its regulatory (R) domain. Unphosphorylated CFTR engages PKA-C at two locations, establishing two “catalytic stations” near to, but not directly involving, the R domain. This configuration, coupled with the conformational flexibility of the R domain, permits transient interactions of the eleven spatially separated phosphorylation sites. Furthermore, we determined two structures of the open-pore CFTR stabilized by PKA-C, providing a molecular basis for understanding ‘reversible activation’, whereby PKA-C stimulates CFTR function through simple binding.