Massachusetts Institute of Technology. Department of Biology, Whitehead Institute for Biomedical Research, Knockenhauer, Kevin E., Schwartz, Thomas, Ploegh, Hidde, Ingram, Jessica R., Markus, Benedikt M., Mandelbaum, Joseph, Ramek, Alexander, Shan, Yibing, Shaw, David E., Lourido, Sebastian, Knockenhauer, Kevin Edward, Massachusetts Institute of Technology. Department of Biology, Whitehead Institute for Biomedical Research, Knockenhauer, Kevin E., Schwartz, Thomas, Ploegh, Hidde, Ingram, Jessica R., Markus, Benedikt M., Mandelbaum, Joseph, Ramek, Alexander, Shan, Yibing, Shaw, David E., Lourido, Sebastian, and Knockenhauer, Kevin Edward
Calcium-dependent protein kinases (CDPKs) comprise the major group of Ca[superscript 2+]-regulated kinases in plants and protists. It has long been assumed that CDPKs are activated, like other Ca[superscript 2+]-regulated kinases, by derepression of the kinase domain (KD). However, we found that removal of the autoinhibitory domain from Toxoplasma gondii CDPK1 is not sufficient for kinase activation. From a library of heavy chain-only antibody fragments (VHHs), we isolated an antibody (1B7) that binds TgCDPK1 in a conformation-dependent manner and potently inhibits it. We uncovered the molecular basis for this inhibition by solving the crystal structure of the complex and simulating, through molecular dynamics, the effects of 1B7–kinase interactions. In contrast to other Ca[superscript 2+]-regulated kinases, the regulatory domain of TgCDPK1 plays a dual role, inhibiting or activating the kinase in response to changes in Ca[superscript 2+] concentrations. We propose that the regulatory domain of TgCDPK1 acts as a molecular splint to stabilize the otherwise inactive KD. This dependence on allosteric stabilization reveals a novel susceptibility in this important class of parasite enzymes., National Institutes of Health (U.S.) (Grant T32GM007287), National Science Foundation (U.S.). Graduate Research Fellowship (Grant 1122374)