Molecular cloning of cDNA for caltractin, a basal body-associated Ca2+-binding protein: homology in its protein sequence with calmodulin and the yeast CDC31 gene product.

An extended synthetic oligonucleotide (59-mer) was used to isolate a Chlamydomonas cDNA containing the entire coding region for a novel basal body-associated 20-kD calcium-binding protein (CaBP). DNA and RNA blot analysis indicate that the 20-kD CaBP is encoded by a single copy gene from which is derived an approximately 1.1-kb-long transcript. The deduced amino acid sequence for the protein shows a linear relatedness with calmodulin from Chlamydomonas and other organisms (45-48% identity). The primary protein sequence of the 20-kD CaBP and its predicted secondary structure suggests that the protein is likely to contain four homologous calcium-binding domains that conform to the helix-loop-helix (or EF hand) structure found in calmodulin and related calcium-modulated proteins. The major difference between the protein and calmodulin is an amino-terminal domain of 21 amino acids present on the 20-kD CaBP. In addition to its relatedness to calmodulin, the Chlamydomonas 20-kD CaBP shows a strong sequence identity (50%) with the yeast Saccharomyces cerevisiae CDC31 gene product required for spindle pole body duplication. The association of these sequence-related calcium-binding proteins to microtubule-organizing centers of divergent structure suggests a potential conserved function for the proteins.