A plant kinesin heavy chain-like protein is a calmodulin-binding protein

Calmodulin, a calcium modulated protein, regulates the activity of several proteins that control cellular functions. A cDNA encoding a unique calmodulin-binding protein, PKCBP, was isolated from a potato expression library using protein-protein interaction based screening. The cDNA encoded protein bound to biotinylated calmodulin and ³⁵S-labeled calmodulin in the presence of calcium and failed to bind in the presence of EGTA, a calcium chelator. The deduced amino acid sequence of the PKCBP has a domain of about 340 amino acids in the C-terminus that showed significant sequence similarity with the kinesin heavy chain motor domain and contained conserved ATP- and microtubule-binding sites present in the motor domain of all known kinesin heavy chains. Outside the motor domain, the PKCBP showed no sequence similarity with any of the known kinesins, but contained a globular domain in the N-terminus and a putative coiled-coil region in the middle. The calmodulin-binding region was mapped to a stretch of 64 amino acid residues in the C-terminus region of the protein. The gene is differentially expressed with the highest expression in apical buds. A homolog of PKCBP from Arabidopsis (AKCBP) showed identical structural organization indicating that kinesin heavy chains that bind to calmodulin are likely to exist in other plants. This paper presents evidence that the motor domain has microtubule stimulated ATPase activity and binds to microtubules in a nucleotide-dependent manner. The kinesin heavy chain-like calmodulin-binding protein is a new member of the kinesin superfamily as none of the known kinesin heavy chains contain a calmodulin-binding domain. The presence of a calmodulin-binding motif and a motor domain in a single polypeptide suggests regulation of kinesin heavy chain driven motor function(s) by calcium and calmodulin.