A Novel Plant Calmodulin-binding Protein with a Kinesin Heavy Chain Motor Domain (*)

Abstract

Calmodulin, a ubiquitous calcium-binding protein, regulates many diverse cellular functions by modulating the activity of the proteins that interact with it. Here, we report isolation of a cDNA encoding a novel kinesin-like calmodulin-binding protein (KCBP) from Arabidopsis using biotinylated calmodulin as a probe. Calcium-dependent binding of the cDNA-encoded protein to calmodulin is confirmed by S-labeled calmodulin. Sequence analysis of a full-length cDNA indicates that it codes for a protein of 1261 amino acids. The predicted amino acid sequence of the KCBP has a domain of about 340 amino acids in the COOH terminus that shows significant sequence similarity with the motor domain of kinesin heavy chains and kinesin-like proteins and contains ATP and microtubule binding sites typical of these proteins. Outside the motor domain, the KCBP has no sequence similarity with any of the known kinesins, but contains a globular domain in the NH terminus and a putative coiled-coil region in the middle. By analyzing the calmodulin binding activity of truncated proteins expressed in Escherichia coli, the calmodulin binding region is mapped to a stretch of about 50 amino acid residues in the COOH terminus region of the protein. Using a synthetic peptide, the calmodulin binding domain is further narrowed down to a 23-amino acid stretch. The synthetic peptide binds to calmodulin with high affinity in a calcium-dependent manner as judged by electrophoretic mobility shift assay of calmodulin-peptide complex. The KCBP is coded by a single gene and is highly expressed in developing flowers and suspension cultured cells. Although many kinesin heavy chains and kinesin-like proteins have been extensively characterized at the biochemical and molecular level in evolutionarily distant organisms, none of them is known to bind calmodulin. The plant kinesin-like protein with a calmodulin binding domain and a unique amino-terminal region is a new member of the kinesin superfamily. The presence of a calmodulin-binding motif in a kinesin heavy chain-like protein suggests a role for calcium and calmodulin in kinesin-driven motor function(s) in plants.