Fig. 1. Domain organization of calpains from arthropods and nematode. Calpains from land crab (Gl-CalpM, Gl-CalpB and Gl-CalpT), Drosophila melanogaster (Dm-CalpA and Dm-CalpB) and Caenorhabditis elegans (TRA-3) are depicted. All calpains share conserved proteolytic (II) and C₂-like (III) domains. The N-terminal domain I varies in sequence and length among different calpains. Calpains differ in the C-terminal region. `Typical' calpains (e.g. Gl-CalpB, Dm-CalpA and Dm-CalpB) have a calmodulin-like domain (IV) containing five EF-hand motifs. `Atypical' calpains either lack domain IV (Gl-CalpM and Ha-CalpM) or have domain IV replaced with a T domain (Gl-CalpT and TRA-3). Gl-CalpM' and Dm-CalpA' are truncated proteins resulting from alternative mRNA splicing of Gl-CalpM and Dm-CalpA, respectively. Amino acid residues, numbered from the N-terminus, indicate the boundaries between the domains.