Crystal structure of a mu-like calpain reveals a partially activated conformation with low Ca2+ requirement

The two Ca2+-dependent cysteine proteases, mu- and m-calpain, are involved in various Ca2+-linked signal pathways but differ markedly in their Ca2+ requirements for activation. We have determined the structure of a mu-like calpain, which has 85% mu-calpain sequence (the first 48 and the last 62 residues of the large subunit are those from m-calpain) and a low Ca2+ requirement. This construct was used because mu-calpain itself is too poorly expressed. The structure of mu-like calpain is very similar in overall fold to that of m-calpain as expected, but differs significantly in two aspects. In comparison with m-calpain, the catalytic triad residues in mu-like calpain, His and Cys, are much closer together in the absence of Ca2+, and significant portions of the Ca2+ binding EF-hand motifs are disordered and more flexible. These structural differences imply that Ca2+-free mu-calpain may represent a partially activated structure, requiring lower Ca2+ concentration to trigger its activation.