Origins of the difference in Ca2+ requirement for activation of mu- and m-calpain.
The mu- and m-calpains are closely related Ca(2+)-dependent cysteine proteases having different in vitro Ca(2+) requirements ( K (d)), of approx. 25 and 325 microM respectively. The two isoforms are heterodimers of slightly different large (80 kDa) subunits and an identical small (28 kDa) subunit, so that the difference in K (d) values must reside in the large subunits. As assayed here, these K (d) values relate to the Ca(2+) required for the first phase of calpain activation and do not reflect the lower Ca(2+) then required by fully activated calpain. On the basis of sequence comparison and the X-ray structure of m-calpain, many m-type residues in the C-terminal EF-hand-containing domain IV were converted into the corresponding mu-type residues, but these mutations did not produce the expected decrease in K (d). In a series of hybrid (mu/m) large-subunit calpains, the K (d) values decreased progressively towards that of mu-calpain as the proportion of mu-type sequence increased from 0 to 90%. K (d) values cannot therefore be ascribed to one or a few specific intramolecular interactions, but reflect the global response of the whole molecule to Ca(2+) binding. Nonetheless, 25% of the difference in K (d) values between mu- and m-calpain can be ascribed to the N-terminal peptide of the large subunit, whereas the C-terminal EF-hand-containing domain IV accounts for 65% of the difference.
Dutt
Previn
P
Department of Biochemistry, Queen's University, Kingston, ON, Canada K7L 3N6.
Spriggs
Cherie N
CN
Davies
Peter L
PL
Jia
Zongchao
Z
Elce
John S
JS
D000595:Amino Acid Sequence; D000818:Animals; D001665:Binding Sites; D002118:Calcium / Q000378:metabolism*; D002154:Calpain / Q000235:genetics / Q000378:metabolism*; D002364:Caseins / Q000378:metabolism; D018076:DNA, Complementary / Q000378:metabolism; D004305:Dose-Response Relationship, Drug; D004926:Escherichia coli / Q000378:metabolism; D007700:Kinetics; D008969:Molecular Sequence Data; D016297:Mutagenesis, Site-Directed; D009154:Mutation; D010957:Plasmids / Q000378:metabolism; D017434:Protein Structure, Tertiary; D051381:Rats; D017386:Sequence Homology, Amino Acid
D002364:Caseins; D018076:DNA, Complementary; D002154:Calpain; C449000:m-calpain; C449002:mu-calpain; D002118:Calcium