N-Terminal domains of cardiac troponin-I modulate thin filament structure

At low-Ca2+, C-terminal domains of troponin subunit-I (TnI) trap tropomyosin in a position on thin filaments that interferes with myosin-binding, thus causing muscle relaxation. This steric inhibition is reversed at high-Ca2+ when TnI releases from F-actin-tropomyosin as Ca2+ and the TnI switch-peptide bind to the N-lobe of troponin-C (TnC). The opposite end of cardiac TnI contains a phosphorylation-sensitive ∼30 residue-long N-terminal peptide that is absent in skeletal muscle, and which can modify these interactions in hearts.