Calmodulin

Calmodulin (CaM) is a small Ca²⁺-binding protein that acts to transduce second messenger signals into a wide array of cellular responses. The primary structures of CaM proteins generally are highly conserved across all lines of eukaryotic phylogeny.  However, a surprising development is the discovery that numerous isoforms of CaM may occur within a single plant species.  Figure 1 aligns the amino acid sequences of typical plant, fern, moss, algal, fungal and vertebrate CaMs.  Cloned cDNA sequences initially were used to deduce the amino acid sequences of these proteins. Although higher plant CaM proteins are highly conserved every plant surveyed thus far harbors genes encoding slightly different isoforms of CaM (for example, there are 7 genes in Arabidopsis encoding 4 amino acid sequence variants of CaM).  This Table lists the CaM genomic and cDNA sequences cloned and sequenced by the Arabidopsis genome project and by our lab.  Full-length cDNA clones and genomic clones are available through the Arabidopsis Biological Resources Center (ABRC).  T-DNA insertional knockouts of most of the genes are available through ABRC provided by a variety of research groups world-wide. 

With the exception of yeast CaM, all CaMs possess four functional EF-hand Ca²⁺-binding domains and highly conserved structures.  The Ca²⁺-binding domains are numbered I through IV, beginning from the amino-terminus.  Figure 2 shows the structure of Ca²⁺-loaded CaM, as deduced by x-ray crystallography; in this form the protein has a dumbbell-shaped structure, with two Ca²⁺-binding sites located at either end of the molecule. 

CaM acts by binding to short peptide sequences within target proteins usually ranging from 10 to 30 amino acids thereby inducing structural changes in the target protein. Often, this change displaces an autoinhibitory domain from the active site of an enzyme. CaM may compete with another signaling molecule (for example, a cyclic nucleotide) for a binding site on a target protein. As a consequence, CaM binding alters target protein activity in response to changes in intracellular Ca²⁺ concentration.  To facilitate this interaction, the central region of CaM forms a random coil. When it does, the globular domains at the N- and C- terminus of the molecule wrap around and engulf the target peptide as shown in Figure 3. 

Because CaM is ubiquitously expressed and has no enzymatic activity of its own, the signaling pathways initiated by CaM and the physiological responses they elicit are derived from the expression patterns and activities of the proteins regulated by CaM.  Identifying these proteins represents an area of intensive current interest in all eukaryotes.  This Table lists the proteins in plants that have been shown to bind or whose activities are regulated by CaM.  As is the case in animal systems, a wide variety of enzymes, transport proteins, and cytoskeletal elements are represented among plant CaM-binding proteins, but a growing number of CaM-regulated proteins in plants appear to be unique. 

We are trying to determine whether there are different physiological roles (i.e., distinctive biochemical activities regulated by) for CaM isoforms.  To do this, we have constructed expression vectors designed to produce CaM in E. coli, whose activities we can study in vitro.

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This page was constructed by Ray Zielinski and last updated on 13 August 2010.