Phytocalpains: orthologous calcium-dependent cysteine proteinases.

A single calcium-dependent cysteine protease (calpain) gene, essential for aleurone cell development, has been identified recently in maize, although this activity had been described previously in Arabidopsis and maize roots associated with anoxia-induced root-tip death. Calpain genes are ubiquitous in animals and there are up to 12 paralogous genes in humans that exhibit molecular diversity outside of their catalytic domain. Calpain orthologous genes have been identified in 11 plant species. Like their animal counterparts, phytocalpains have significant homology within the catalytic domain, but lack the conserved calcium-binding domain IV, and some members have an N-terminal transmembrane receptor-like domain.