In the mammalian calpain system, two isozymes, mu- and m-types, have been well-characterized, and are considered to be conserved in the avian system as well. Thus, chicken calpain, whose large subunit was cloned in 1984, has long been regarded as 'm-type', since chicken also possesses 'mu-type' activity, although its structure has not yet been elucidated. In this study, we identified three kinds of cDNAs encoding distinct chicken calpain large subunits. Two of the three were highly similar to the mammalian mu-type and p94, respectively. The third shows a much higher similarity to mammalian m-type than the first identified chicken calpain, indicating that this molecule, which has been considered as 'm-type', should be renamed. We, therefore, designated it 'mu/m-calpain', because its sequence and Ca(2+)-sensitivity lie between mu- and m-types. Northern blot analyses revealed that chicken mCL and muCL, as well as mu/mCL, show ubiquitous expression, while p94 was detected predominantly in skeletal muscle, as previously reported. Chicken skeletal muscle, therefore, expresses at least four types of calpain, three ubiquitous and one tissue-specific.
