A novel human small subunit of calpains.

Typical calpains are heterodimeric cysteine proteases which have distinct large catalytic subunits (80 kDa) but share a common small regulatory subunit (30 kDa; css1). Here we report the identification, cloning and characterization of a novel human small subunit (css2) encoded by an intronless gene, capns2, located on chromosome 16. This new protein displays 73% sequence identity within the Ca(2+)-binding region but lacks two oligo-Gly stretches characteristic of the N-terminal domain of the conventional small subunit. css2 appears to be the functional equivalent of the conventional small subunit in vitro in that it helps the large subunit fold into the active conformation of similar Ca(2+) sensitivity when the two proteins are co-expressed in Escherichia coli. The purification of various chimaeric rat 80 kDa-human css2 constructs, on the other hand, shows that css2 binds the large subunit much more weakly than css1. Further, it does not undergo the autolytic conversion typical of the classical small subunit. The expression of this protein in vivo, as assessed from its appearance in expressed sequence tag clones, is rather limited, making it an example of a tissue-specific, rather than ubiquitous, small subunit.