Structural consequences of cardiac troponin I phosphorylation.
beta-Adrenergic stimulation of the heart results in bisphosphorylation of the N-terminal extension of cardiac troponin I (TnI). Bisphosphorylation of TnI reduces the affinity of the regulatory site on troponin C (TnC) for Ca(2+) by increasing the rate of Ca(2+) dissociation. What remains unclear is how the phosphorylation signal is transmitted from one subunit of troponin to another. We have produced a series of mutations in the N-terminal extension of TnI designed to further our understanding of the mechanisms involved. The ability of phosphorylation of the mutant TnIs to affect Ca(2+) sensitivity has been assessed. We find that the Pro residues found in a conserved (Xaa-Pro)(4) motif N-terminal to the phosphorylation sites are not required for the effect of the N-terminal extension on Ca(2+) binding in the presence or absence of phosphorylation. Our experiments also reveal that the full effects of phosphorylation are seen even when residues 1-15 of TnI are deleted. If further residues are removed, not only does the effect of phosphorylation diminish but deletion of the N-terminal extension mimics phosphorylation. We propose that TnI residues 16-29 bind to TnC stabilizing the "open" Ca(2+)-bound state. Phosphorylation (or deletion) prevents this binding, accelerating Ca(2+) release.
School of Biosciences, University of Birmingham, Edgbaston, United Kingdom.
Douglas G
DG
Ward
Michael P
MP
Cornes
Ian P
IP
Trayer
D000595:Amino Acid Sequence; D002118:Calcium / Q000378:metabolism; D006801:Humans; D008969:Molecular Sequence Data; D009206:Myocardium / Q000737:chemistry*; D010766:Phosphorylation; D011487:Protein Conformation; D020033:Protein Isoforms; D019209:Troponin C / Q000737:chemistry; D019210:Troponin I / Q000737:chemistry* / Q000378:metabolism; D020107:Troponin T / Q000737:chemistry
D020033:Protein Isoforms; D019209:Troponin C; D019210:Troponin I; D020107:Troponin T; D002118:Calcium