Calcium regulatory proteins in plants; an NMR approach

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Calcium plays an important role as a secondary messenger in all eukaryotic cells. An increase in the cytosolic Ca2+ level can act like a trigger device by binding to regulatory calcium-binding proteins, such as calmodulin (CaM). Collectively the calcium-sensor proteins can redirect the physiological status of a cell. In plants the cytosolic Ca2+ level increases in response to various abiotic and biotic stresses such as cold, drought, salt, wounding or the presence of plant pathogens. We have recently reported the solution structures of various plant CaM's and CaM-like protein domains in their apo and Ca2+-saturated states. Moreover we have also determined three high-resolution structures of complexes of a CaM isoform with three plant proteins that are directly involved in mediating the stress response: glutamate decarboxylase, MAPK-phosphatase and a vacuolar calcium-ATPase. In all three cases novel complex structures were obtained, which had not been seen before for any mammalian CaM complexes, highlighting unexpected structural diversity of the plant calcium-sensor protein family. Recent genome sequencing efforts have revealed that all plants possess a large variety of CaM's, CaM-like proteins and CaM-like protein domains. For example, plants (and protozoa) contain a unique class of protein kinases (commonly known as the CDPK's and the CCaMK's) that are not found in other organisms. These plant-specific enzymes contain their own integrated CaM-like domains, which modulate their activity. We plan to solve the three-dimensional structures of some of the most distinct regulatory domains to find out how they differ in the way they interact with calcium, their protein targets, or other calcium-sensor proteins. Moreover we will complete our studies of a highly divergent CaM-like protein that plays a role in the salt stress response in plants. Finally we will study an unusual membrane anchored version of CaM that has been detected in various plants. For this research program we will continue to use Nuclear Magnetic Resonance (NMR) spectroscopy as our main -but not exclusive- experimental tool, to obtain detailed structural and dynamic information about these proteins and their complexes in the solution state.