Structure of the regulatory apparatus of a calcium-dependent protein kinase (CDPK): a novel mode of calmodulin-target recognition.

Calcium-dependent protein kinases (CDPKs) are a class of calcium-binding sensory proteins that are found in plants and certain protozoa, including the causative agent of malaria, Plasmodium falciparum . CDPKs have diverse regulatory functions, including involvement in the triggering of the lytic cycle of malarial infection. CDPKs contain an autoinhibitory junction (J) region whose calcium-dependent interaction with the tethered regulatory calmodulin-like domain (CaM-LD) activates the catalytic kinase domain. We report here the X-ray crystal structure of the J-CaM-LD region of CDPK from Arabidopsis thaliana (AtCPK1), determined to 2.0 A resolution using multiple-wavelength anomalous dispersion (MAD). The structure reveals a symmetric dimer of calcium-bound J-CaM-LD with domain-swap interactions, in which the J region of one protomer interacts extensively with the carboxy-terminal EF-hand domain (C-lobe) of the partner protomer. However, as the J-CaM-LD is monomeric in solution, the activated monomer was modelled to account for the intra-molecular recognition of the two domains. While the J-CaM-LD segment mimics certain aspects of target motif recognition by CaM other features are specific to CDPKs, in particular the combination of the strong interaction between the N and C-lobes of the CaM-LD and the exclusive use of only the C-lobe in the recognition of the covalently tethered target region. Combined with our previous observations showing that there is likely to be strong interactions between this tethered J region and the CaM-LD even at basal Ca 2+ concentrations, the new structural data indicate that the response to calcium of CDPKs is clearly unique among the CaM family.