Jy. Cong et al., EFFECT OF MONOCLONAL-ANTIBODIES SPECIFIC FOR THE 28-KDA SUBUNIT ON CATALYTIC PROPERTIES OF THE CALPAINS, The Journal of biological chemistry, 268(34), 1993, pp. 25740-25747
Nine monoclonal antibodies (mAbs) specific for the 28-kDa subunit comm
on to mu- and m-calpains have been assayed for their effects on mu- an
d m-calpains. All nine react with the COOH-terminal part (domain VI) o
f the 28-kDa subunit, and all nine affect the Ca2+ concentration requi
red for autolysis of m-calpain, but have little effect on the Ca2+ con
centration required for autolysis of mu-calpain. None of the nine affe
ct the specific proteolytic activity of mu- or m-calpain. Two of the m
Abs, 5B9 and 5B3, were selected for further study. mAb 5B9 decreased t
he Ca2+ concentration required for autolysis to one-fifth of that requ
ired in its absence; sequencing of chymotryptic fragments showed that
the epitope for mAb 5B9 is between amino acid residues 92 and 104 of t
he 28-kDa subunit. mAb 5B3 increased the Ca2+ concentration required f
or autolysis; the epitope for mAb 5B3 is located between amino acid re
sidues 148 and 178 of the 28-kDa subunit, which is the region that con
tains the first EF-hand Ca2+-binding sequence in this subunit. Althoug
h it increases the Ca2+ concentration required for autolysis, mAb 5B3
has no effect on the Ca2+ concentration required for proteolytic activ
ity of m-calpain, and unautolyzed m-calpain is not a proenzyme. That a
ll nine mAbs react with domain VI and not with the NH2-terminal domain
V of the 28-kDa subunit suggests that domain VI (and not domain V) is
involved in autolysis, contrary to the view that phosphatidylinositol
lowers the Ca2+ concentration required for autolysis by binding to do
main V.