Their
cDNA-derived amino acid sequences predict that the 80-kDa subunits of the micromolar and millimolar Ca(2+)-requiring forms of the Ca(2+)-dependent
proteinase (mu- and
m-calpain, respectively) each consist of four domains and that the 28-kDa subunit common to both mu- and
m-calpain consists of two domains. The calpains were allowed to autolyze to completion, and the
autolysis products were separated and were characterized by using gel permeation chromatography,
calpastatin affinity chromatography, and sequence analysis. Three major fragments were obtained after
autolysis of either
calpain. The largest fragment (34 kDa for
mu-calpain, 35 kDa for
m-calpain) contains all of domain II, the catalytic domain, plus part of domain I of the 80-kDa subunit of mu- or
m-calpain. This fragment does not bind to
calpastatin, a competitive inhibitor of the calpains, and has no proteolytic activity in either the absence or presence of Ca2+. The second major fragment (21 kDa for
mu-calpain and 22 kDa for
m-calpain) contains domain IV, the
calmodulin-like domain, plus approximately 50
amino acids from domain III of the 80-kDa subunit of mu- or
m-calpain. The third major fragment (18 kDa) contains domain VI, the
calmodulin-like domain of the 28-kDa subunit. The second and third major fragments bind to a
calpastatin affinity column in the presence of Ca2+ and are eluted with
EDTA. The second and third fragments are noncovalently bound, so the 80- and 28-kDa subunits of the intact
calpain molecules are noncovalently bound at domains IV and VI. After separation in 1 M
NaSCN, the 28-kDa subunit binds completely to
calpastatin, approximately 30-40% of the 80-kDa subunit of
mu-calpain binds to
calpastatin, and the 80-kDa subunit of
m-calpain does not bind to
calpastatin in the presence of 1 mM Ca2+.