Isolation, characterization, and bioinformatic analysis of calmodulin-binding protein cmbB reveals a novel tandem IP22 repeat common to many Dictyostelium and Mimivirus proteins

A novel calmodulin-binding protein cmbB from Dictyostelium discoideum is encoded in a single gene. Northern analysis reveals two cmbB transcripts first detectable at 4 h during multicellular development. Western blotting detects an ∼46.6 kDa protein. Sequence analysis and calmodulin–agarose binding studies identified a “classic” calcium-dependent calmodulin-binding domain (179IPKSLRSLFLGKGYNQPLEF198) but structural analyses suggest binding may not involve classic α-helical calmodulin-binding. The cmbB protein is comprised of tandem repeats of a newly identified IP22 motif ([I,L]Pxxhxxhxhxxxhxxxhxxxx; where h = any hydrophobic amino acid) that is highly conserved and a more precise representation of the FNIP repeat. At least eight Acanthamoeba polyphaga Mimivirus proteins and over 100 Dictyostelium proteins contain tandem arrays of the IP22 motif and its variants. cmbB also shares structural homology to YopM, from the plague bacterium Yersenia pestis.