The study of calpains has a long history, starting in 1964 when Gordon Guroff first discovered calpain in the brain. Since then, many researchers have given numerous names to a variety of calpain molecules as well as their component domains. Thus, a common nomenclature for the calpains and their domains needs to be established.
For the calpain domains, there are at least two parallel systems in use. Here we propose that the domain numbers (I-VI) be replaced by descriptive acronyms (see “Conventional calpains”, and Fig. 1). In addition, although this is a minor detail, we have suggested a general color scheme for the domains that follows the structural biology tradition of using the colours of the rainbow to indicate distance along the polypeptide chain (see Fig. 1, for example).
The nomenclature for the calpains themselves varies depending on the organisms, and should be determined for each organism by its experts. In general, the nomenclature systems for genes and their products are well established these days, and it would be best to follow these rules for the calpain nomenclature. Therefore, a gene-based nomenclature system is introduced for each representative organism (We hope the number of examples will increase with input from calpain researchers studying different organisms!).
Another complex issue is the nomenclature for calpain enzymes having the tertiary structure. It has been proposed that the name "calpain" should be reserved for enzymes, not subunits, and that "μ-" and/or "m-" nomenclature is confusing. Thus, simply, μ-calpain would be calpain-1 (also described with a subunit compsition, "CAPN1/S1", a short form for "CAPN1/CAPNS1") and m-calpain would be calpain-2 (CAPN2/S1).