Calcium regulatory proteins in plants and yeasts; 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 by binding to regulatory calcium-binding proteins, such as calmodulin (CaM). Collectively upon binding Ca2+ these '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. As such Ca2+ is an important mediator of the plant stress response. Recent genome sequencing efforts have shown that all plants contain a large diversity of calcium-sensor proteins such as distinct CaM's, numerous CaM-like proteins as well as a variety of protein kinases and other systems that possess their own integrated CaM-like protein domains. For example, plants (and protozoa) contain a unique class of calcium binding protein kinases (commonly known as the CDPK's and CCaMK's) that are not found in other organisms. My research group has been interested in mapping out the molecular details of the calcium-sensor proteins and protein domains that participate in the plant stress response. The main experimental technique that we rely upon in this research program is Nuclear Magnetic Resonance (NMR) spectroscopy, which allows us to obtain detailed structural and dynamic information about these proteins and their complexes in their natural solution state. In addition we make use of various other biophysical tools to study the kinetics and energetics of the protein-calcium and protein-protein interactions. To date, we have determined the structures of the CaM-like domains of various plant protein kinases, as well as studied the properties of plant CaMs and their complexes with various enzymes and transmembrane calcium pumps. In the near future we want to study three selected CaM-like proteins that are directly involved in the plant stress response. Moreover, we plan to study how calcium-CaM can inhibit cyclic-nucleotide gated channels, that allow Ca2+ influx in the plant cells, by binding to a regulatory cytoplasmic domain of these channels. In addition, we will focus our attention on plant (and related yeast) fimbrins, a group of proteins that play an important role in plant growth. These proteins have been highly conserved during evolution and they are made up two calcium-binding motifs as well as four actin-binding CH-domains. Finally we plan to study a transport protein that is embedded in the membranes that surround the mitochondria. This protein specifically transports ATP-Mg2+ into the mitochondrial interior and it is regulated by Ca2+ through a unique CaM-like structure, that resides on top of the transmembrane channel. The knowledge gained through these projects may help in the future to improve the drought or salt-resistance of important crops or can help us develop new herbicides in a rational manner.