Modulation of the Interaction between Troponin I N-Terminal Peptide and Troponin C by Phosphorylation Studied by Molecular Dynamics

Abstract

The Ca2+-sensitivity of cardiac thin filaments is modulated by interaction between the N terminal peptide of troponin I and the N terminal lobe of troponin C that enhances Ca2+ sensitivity. The interaction is abolished by PKA phosphorylation of Ser 22 and 23. We have applied molecular dynamics simulations to the Takeda et al. structure of the core domain of human cardiac troponin in explicit water on an expanded model of the full crystallographic structure (385 amino acids). The crystal structure is deficient in the first 31 residues of TnI which we have added in as a linear chain above the TnC N-terminal lobe according to the model of Howarth et al., (J Mol Biol 373 , 709). All simulations have been performed for a quarter of a microsecond with the AMBER GPU MD package in an isobaric-isothermal, NPT, ensemble.In the Ca2+-bound unphosphorylated state there is a persistent interaction between Arg 20 through Ser 24 of TnI and the Ca2+-binding loops of Tn-C that could modulate Ca2+-binding. The extreme N-terminal 1 to 16 Amino acids of TnI are very mobile.Phosphorylation leads to restructuring after 50 nsecs with reduction of strong TnI-TnC contacts and a re-orientation of the TnC N-terminal lobe relative to the rest of troponin. Ca2+ becomes more exposed to solvent. Between 100 and 150ns the Ca2+ dissociates and reassociates. Between 150 and 200ns the first 20 amino acids of TnI form a beta sheet structure that interacts with alpha helices Lys39 - Leu 48 and Pro 54 and Ile 61 of TnC. The adjacent TnI 43-59 helix that normally binds in a cleft of the C-terminal lobe of troponin C is also perturbed, indicating long range effects of phosphorylation.