Structural Characterization of a Higher Plant Calmodulin

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Abstract
Calmodulin is a eukaryotic Ca binding protein which has several Ca-dependent in vitro activities. A structural characterization of calmodulin from spinach leaves is reported. Spinach calmodulin may be representative of higher plant calmodulins in general since calmodulin from the monocotyledon barley (Hordeum vulgare) is indistinguishable by a variety of physical, chemical and functional criteria. Spinach calmodulin is homologous to bovine brain calmodulin with only 13 identified amino acid sequence differences, excluding a blocked NH2-terminal tripeptide whose sequence has not been elucidated. Two extended regions of sequence identity are in the NH2-terminal half of the molecule, while 9 of the 13 identified differences are in the COOH-terminal half of the molecule. Two of the changes, a cysteine at residue 26 and a glutamine at residue 96, require a minimum of 2 base changes in the nucleotide codons. Both of these changes occur in proposed Ca binding loops of the molecule. Five additional amino acid differences found in spinach calmodulin had not been observed previously in a calmodulin. These limited number of amino acid sequence variations appear to result in differential effects on the activation of calmodulin-dependent enzymes by plant and vertebrate calmodulins.
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