The effects of truncations of the small subunit on m-calpain activity and heterodimer formation

In order to study subunit interactions in calpain, the effects of small subunit truncations on m-calpain activity and heterodimer formation have been measured. It has been shown previously that active calpain is formed by co-expression of the large subunit (80 kDa) of rat m-calpain with a δ86 form (21 kDa) of the small subunit. cDNA for the full-length 270 amino acid (28.5 kDa) rat calpain small subunit has now been cloned, both with and without an N-terminal histidine tag (NHis₁₀). The full-length small subunit constructs yielded active calpains on co-expression with the large subunit, and the small subunit was autolysed to 20 kDa on exposure of these calpains to Ca²⁺. A series of deletion mutants of the small subunit, NHis₁₀-δ86. δ99, -δ107, and -δ116, gave active heterodimeric calpains with unchanged specific activities, although in decreasing yield, and with a progressive decrease in stability. NHis₁₀-δ125 formed a heterodimer which was inactive and unstable. Removal of 25 C-terminal residues from Δ86, leaving residues 87-245, abolished both activity and heterodimer formation. The results show that: (a) generation of active m-calpain in Escherichia coli requires heterodimer formation; (b) small subunit residues between 94 and 116 contribute to the stability of the active heterodimer but do not directly affect the catalytic mechanism; (c) residues in the region 245-270 are essential for subunit binding. Finally, it was shown that an inactive mutant Cys¹⁰⁵→Ser-80k/δ86 calpain, used in order to preclude autolysis, did not dissociate in the presence of Ca²⁺, a result which does not support the proposal that Ca²⁺-induced dissociation is involved in calpain activation.