title: Cardiac troponin I tyrosine 26 phosphorylation decreases myofilament Ca2+ sensitivity and accelerates deactivation.
abstract: Troponin I (TnI), the inhibitory subunit of the troponin complex, can be phosphorylated as a key regulatory mechanism to alter the calcium regulation of contraction. Recent work has identified phosphorylation of TnI Tyr-26 in the human heart with unknown functional effects. We hypothesized that TnI Tyr-26N-terminal phosphorylation decreases calcium sensitivity of the thin filament, similar to the desensitizing effects of TnI Ser-23/24 phosphorylation. Our results demonstrate that Tyr-26 phosphorylation and pseudo-phosphorylation decrease calcium binding to troponin C (TnC) on the thin filament and calcium sensitivity of force development to a similar magnitude as TnI Ser-23/24 pseudo-phosphorylation. To investigate the effects of TnI Tyr-26 phosphorylation on myofilament deactivation, we measured the rate of calcium dissociation from TnC. Results demonstrate that filaments containing Tyr-26 pseudo-phosphorylated TnI accelerate the rate of calcium dissociation from TnC similar to that of TnI Ser-23/24. Finally, to assess functional integration of TnI Tyr-26 with Ser-23/24 phosphorylation, we generated recombinant TnI phospho-mimetic substitutions at all three residues. Our biochemical analyses demonstrated no additive effect on calcium sensitivity or calcium-sensitive force development imposed by Tyr-26 and Ser-23/24 phosphorylation integration. However, integration of Tyr-26 phosphorylation with pseudo-phosphorylated Ser-23/24 further accelerated thin filament deactivation. Our findings suggest that TnI Tyr-26 phosphorylation functions similarly to Ser-23/24N-terminal phosphorylation to decrease myofilament calcium sensitivity and accelerate myofilament relaxation. Furthermore, Tyr-26 phosphorylation can buffer the desensitization of Ser-23/24 phosphorylation while further accelerating thin filament deactivation. Therefore, the functional integration of TnI phosphorylation may be a common mechanism to modulate Ser-23/24 phosphorylation function.
authors.affiliation: The Department of Physiology and Cell Biology, The Ohio State University, Columbus, OH 43210, USA; The Davis Heart and Lung Research Institute, The Ohio State University, Columbus, OH 43210, USA.
authors.forename: Hussam E
authors.initials: HE
authors.lastname: Salhi
authors.affiliation: The Department of Physiology and Cell Biology, The Ohio State University, Columbus, OH 43210, USA; The Davis Heart and Lung Research Institute, The Ohio State University, Columbus, OH 43210, USA.
authors.forename: Shane D
authors.initials: SD
authors.lastname: Walton
authors.affiliation: The Department of Physiology and Cell Biology, The Ohio State University, Columbus, OH 43210, USA; The Davis Heart and Lung Research Institute, The Ohio State University, Columbus, OH 43210, USA.
authors.forename: Nathan C
authors.initials: NC
authors.lastname: Hassel
authors.affiliation: The Department of Physiology and Cell Biology, The Ohio State University, Columbus, OH 43210, USA; The Davis Heart and Lung Research Institute, The Ohio State University, Columbus, OH 43210, USA.
authors.forename: Elizabeth A
authors.initials: EA
authors.lastname: Brundage
authors.affiliation: The Department of Cell and Molecular Physiology, Health Sciences Division, Loyola University Chicago, Maywood, IL 60153, USA.
authors.forename: Pieter P
authors.initials: PP
authors.lastname: de Tombe
authors.affiliation: The Department of Physiology and Cell Biology, The Ohio State University, Columbus, OH 43210, USA; The Davis Heart and Lung Research Institute, The Ohio State University, Columbus, OH 43210, USA.
authors.forename: Paul M L
authors.initials: PM
authors.lastname: Janssen
authors.affiliation: The Department of Physiology and Cell Biology, The Ohio State University, Columbus, OH 43210, USA; The Davis Heart and Lung Research Institute, The Ohio State University, Columbus, OH 43210, USA.
authors.forename: Jonathan P
authors.initials: JP
authors.lastname: Davis
authors.affiliation: The Department of Physiology and Cell Biology, The Ohio State University, Columbus, OH 43210, USA; The Davis Heart and Lung Research Institute, The Ohio State University, Columbus, OH 43210, USA. Electronic address: biesiadecki.1@osu.edu.
authors.forename: Brandon J
authors.initials: BJ
authors.lastname: Biesiadecki
mesh_terms: D000818:Animals; D002118:Calcium / Q000378:metabolism*; D020013:Calcium Signaling; D006329:Heart Conduction System; D006801:Humans; D009200:Myocardial Contraction; D009210:Myofibrils / Q000502:physiology; D010766:Phosphorylation; D011499:Protein Processing, Post-Translational; D051381:Rats; D019210:Troponin I / Q000378:metabolism*; D014443:Tyrosine / Q000378:metabolism
chemical_list: D019210:Troponin I; D014443:Tyrosine; D002118:Calcium
keywords: Calcium sensitivity; Cardiac troponin I; Thin filament deactivation; Tyrosine phosphorylation