CDPK subfamily roles of α C helix arginine and activation-loop threonine. Structures of several different CDPKs in C. parvum, demonstrating several proposed interactions for the αC helix arginine distinctive of an alveolate-specific CDPK subfamily. (A) A member of the background set of CDPKs [PDB:3DFA] has a threonine (T50), shown in cyan, in position to form a hydrogen bond with an aspartate (D47), gray, which caps the αC helix. This threonine corresponds to the subfamily-conserved arginine; however, the threonine here is not conserved in the background set of CDPKs. (B) In a structure of a member of the CDPK subfamily [PDB:2QG5], the subfamily-conserved arginine (R69, cyan) appears similarly positioned to interact with the aspartate (D66, blue) at the end of the αC helix, potentially stabilizing the cap. (C) Chain A of the same structure shows the distinctive arginine oriented inward, capable of hydrogen-bonding with the kinase-conserved DFG motif (side chains colored magenta). (D) In another structure of the same CDPK-subfamily protein [PDB:2QG5], the arginine is positioned toward a subfamily-conserved threonine in the activation loop (T184), shown in cyan. The distance between the R69 and T184 side chains is 6Å, which could accomodate a phosphate group attached to the threonine and a hydrogen bond between the phosphothreonine and the arginine.