p94/calpain 3 is a skeletal muscle-specific Ca(2+)-regulated
cysteine protease (
calpain), and genetic loss of p94
protease activity causes
muscular dystrophy (
calpainopathy). In addition, a small in-frame deletion in the N2A region of
connectin/
titin that impairs p94-connectin interaction causes a severe
muscular dystrophy (mdm) in mice. Since p94 via its interaction with the N2A and M-line regions of
connectin becomes part of the
connectin filament system that serves as a molecular scaffold for the myofibril, it has been proposed that structural and functional integrity of the p94-connectin complex is essential for health and maintenance of myocytes. In this study, we have surveyed the interactions made by p94 and
connectin N2A inside COS7 cells. This revealed that p94 binds to
connectin at multiple sites, including newly identified loci in the N2A and PEVK regions of
connectin. Functionally, p94-N2A interactions suppress p94
autolysis and protected
connectin from proteolysis. The
connectin N2A region also contains a binding site for the muscle ankyrin repeat
proteins (MARPs), a
protein family involved in the cellular stress responses. MARP2/Ankrd2 competed with p94 for binding to
connectin and was also proteolyzed by p94. Intriguingly, a
connectin N2A fragment with the mdm deletion possessed enhanced resistance to
proteases, including p94, and its interaction with MARPs was weakened. Our data support a model in which MARP2-p94 signaling converges within the N2A
connectin segment and the mdm deletion disrupts their coordination. These results also implicate the dynamic nature of
connectin molecule as a regulatory scaffold of p94 functions.