The Structure of Sperm Autoantigenic Protein (SPA17): An R2D2 Protein Critical to Cilia and Implicated in Oncogenesis

Abstract

A-Kinase Anchoring proteins (AKAPs) localize the activity of cyclic AMP (cAMP)-Dependent Protein Kinase (PKA) through interaction of an amphipathic helix that binds to a conserved RIIα docking and dimerization (R2D2) domain on the N-terminus of PKA. Genome analysis indicates that at least thirteen other RIIα superfamily proteins exist in humans, which are not coupled to cyclic nucleotide binding domains and are largely localized to cilia and flagella. The newly reported R2D2 proteins exist in two lineages differing by their similarity to Type I or Type II PKA. Moreover, R2D2 domains bind to AKAPs and can contain extra regulatory sequences conferring novel functions and binding specificity. Here we detail the structure of one such domain comprising the N-terminus of Sperm Autoantigenic Protein 17 (SPA17) resolved to 1.72 Ã . The structure of core hydrophobic sites for dimerization and AKAP binding are highly conserved between PKA and SPA17. Additional flanking sequences outside of the core R2D2 domain occlude the AKAP binding site and reduce the affinity for AKAP helices in the absence of heterodimerization with another R2D2 protein, ROPN1L.