The structure of the NH2-terminal region of
troponin T (
TnT) is hypervariable among the muscle type-specific
isoforms and is also regulated by alternative RNA splicing. This region does not contain binding sites for other thin filament
proteins, but alteration of its structure affects the Ca2+ regulation of muscle contraction. Here we report a truncated cardiac
TnT produced during
myocardial ischemia reperfusion.
Amino acid sequencing and
protein fragment reconstruction determined that it is generated by a posttranslational modification selectively removing the NH2-terminal variable region and preserving the conserved core structure of
TnT.
Triton X-100 extraction of cardiac muscle fibers promoted production of the NH2-terminal truncated cardiac
TnT (cTnT-ND), indicating a myofibril-associated proteolytic activity.
Mu-calpain is a myofibril-associated
protease and is known to degrade
TnT. Supporting a role of
mu-calpain in producing cTnT-ND in
myocardial ischemia reperfusion,
calpain inhibitors decreased the level of cTnT-ND in Triton-extracted myofibrils.
Mu-calpain treatment of the cardiac myofibril and
troponin complex specifically reproduced cTnT-ND. In contrast,
mu-calpain treatment of isolated cardiac
TnT resulted in nonspecific degradation, suggesting that this structural modification is relevant to physiological structures of the myofilament.
Triton X-100 treatment of transgenic mouse cardiac myofibrils overexpressing fast skeletal muscle
TnT produced similar NH2-terminal truncations of the endogenous and exogenous
TnT, despite different amino acid sequences at the cleavage site. With the functional consequences of removing the NH2-terminal variable region of
TnT, the
mu-calpain-mediated proteolytic modification of
TnT may act as an acute mechanism to adjust muscle contractility under stress conditions.