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Figure 14.1 Schematic of the domain structure of m- and m-calpain 80 kDa large subunit
(top) and 30 kDa shared regulatory subunit (bottom). The N-terminal domain i is cleaved
during activation, domains iia and iib contain the protease domain, domain iii contains
lipid binding and phosphorylation sites and domain iv contains EF hands. The regulatory
subunit domain vi is homologous to domain iv of the large subunit and contains EF hands
with a potential role in calcium binding. (A colour reproduction of this figure can be found
in the colour plate section)
There are two ubiquitous calpain isoforms that are defined by their in vitro
requirement of calcium for activation. m-calpain (calpain I) requires calcium
concentrations in the micromolar range for activation and m-calpain (calpain
II) requires calcium concentrations in the millimolar range (Glading et al.,
2002; Perrin and Huttenlocher, 2002). Both isoforms contain an 80 kDa
catalytic subunit and a 30 kDa regulatory domain (Figure 14.1). The
regulatory domain is identical for both m- and m-calpain. A critical role for
calpain in normal embryonic development is supported by the observation
that targeted disruption of the small calpain regulatory subunit in mice results
in embryonic lethality at day 10 (Arthur et al., 2000).
Calpains act on many substrates in vitro including enzymes, cytoskeleton-
associated proteins (talin, filamin, a-actinin) (Beckerle et al., 1987), integrin
cytoplasmic domains (Du et al., 1995) and the signalling molecules protein
kinase C (PKC), focal adhesion kinase (FAK) and Src (Carragher et al., 2002;
Cooray et al., 1996; Glading et al., 2002) (Table 14.1). However, few
physiological substrates for calpain activity in vivo have been identified. There
is recent evidence that talin is an important substrate for calpain activity in
vivo (Dourdin et al., 2001).
Other recent studies suggest that the integrin cytoplasmic domain may be an
important substrate for calpain. For example, a calpain-cleaved form of the b3
integrin cytoplasmic domain is found in peripheral adhesive complexes in
spreading cells (Bialkowska et al., 2000). Calpain cleavage modifies its
substrates rather than performing a degradative function. There is evidence
that calpain, through such modifications, plays an important role in regulating
intracellular signalling pathways and modulates reversible, dynamic processes
such as cell migration (Glading et al., 2002; Inomata et al., 1996). However
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