Protein Phosphorylation and Signal Transduction in Cardiac Thin Filaments

Abstract

Homeostasis of cardiac function requires significant adjustments in sarcomeric protein phosphorylation. The existence of unique peptides in cardiac sarcomeres, which are substrates for a multitude of kinases, strongly supports this concept (1). We focus here on the troponin complex of the thin filaments, which contain two major proteins that participate in these phosphoryl group transfer reactions: the inhibitory protein (cardiac troponin (cTn) 2 I) and the tropomyosin (Tm)-binding protein (cTnT). We describe the relatively new understanding of the molecular mechanisms of thin filament-based control of the heartbeat and how these mechanisms are altered by phosphorylation. We discuss new concepts regarding the relation between the beat of the heart and the location of thin filament proteins and their long-and short-range interactions. We also discuss elucidation of mechanisms by which these phosphorylations exacerbate or ameliorate effects of mutations in the myofilament proteins that are linked to familial cardiomyopathies. Fig. 1 depicts an A-band region of the cardiac thin filament functional unit in the diastolic and systolic states. In diastole, force-generating reactions of cross-bridges with actin are inhibited, ATP hydrolysis is relatively low, and the sarcomere is relatively extensible (2). Properties of the giant protein titin dominate the compliance of the relaxed sarcomere (3). Interactions of thin filament regulatory proteins, the troponin heterotrimeric complex, and Tm hinder the actin-cross-bridge reaction and establish the B-state. Calcium binding to a single regulatory site on cTnC triggers a release from this inhibited state by modifications of interactions among actin, Tm, and Tn. Thin Filaments during the Relaxed StateEvidence derived from the core crystal structure of cardiac Tn (4), from elucidation of the structures by NMR (5), from biochemical investigations of protein-protein interactions (6, 7), and from reconstructions and single-particle analysis of electron micrographs of reconstituted myofilament preparations (8, 9) provided the basis for the illustration in Fig. 1 (see Ref. 6 for a review). Apart from the lack of a thin filament lattice in the Tn core crystal structure, there was no structural information on significant regions, including the tail region of cTnT, an inhibitory peptide (Ip; which tethers cTnI to actin), the unique N-terminal peptide (ϳ30 amino acids), and portions of the far C-terminal domain of cTnI. Thus, Fig. 1 (upper) shows binding of cTnI to actin via two regions, the highly basic Ip and a second actin-binding region. Importantly, these regions flank a switch peptide, which binds to cTnC when Ca 2ϩ binds to the N-terminal lobe of cTnC, which houses the regulatory Ca 2ϩ -binding site, thereby participating in the mechanism by which Tn releases the thin filament from inhibition.The C-terminal mobile domain of cTnI beyond the second actin-binding site may also participate in establishing the relaxed state. Two observations point to this possibility. The first is a ...

Titin strain contributes to the Frank–Starling law of the heart by structural rearrangements of both thin- and thick-filament proteins

“…It is known that Ca 2+ binding induces an opening of a hydrophobic patch on TnC with affinity toward the switch peptide domain of TnI, ultimately resulting in release of TnI from the actin-binding site, freeing up the myosin site so as to initiate muscle contraction (26). Hence, it appears that the structural rearrangement of troponin upon stretch is different from the structural rearrangement of troponin induced by Ca 2+ binding to TnC.…”

Impact of site-specific phosphorylation of protein kinase A sites Ser23and Ser24of cardiac troponin I in human cardiomyocytes

“…A more recent study (16) has provided insights on the conformational transitions within the cTn complex upon bisphosphorylation, but alternative models have been proposed as well (for a recent review, see Ref. 31).…”

“…cTnI-AA, pseudo-dephosphorylated cTnI; cTnI-AD, pseudo-monophosphorylation of Ser 24 ; cTnI-DA, pseudomonophosphorylation of Ser 23 ; cTnI-DD, pseudo-bisphosphorylated cTnI. 31.0 Ϯ 4.6% monophosphorylated cTnI, and 1.8 Ϯ 0.5% unphosphorylated cTnI in the three donor hearts used in the exchange experiments (Fig. 2B), whereas unphosphorylated (60.3%) and monophosphorylated (33.1%) cTnI were the most abundant forms in the IDCM sample used ( Fig.…”

Changes in myofilament proteins, but not Ca2+ regulation, are associated with a high-fat diet-induced improvement in contractile function in heart failure

“…Likewise, the phosphorylation of other myofilament proteins, e.g., myosin-binding protein C (MyBP-C), troponin I, and myosin regulatory light chain (MRLC), have also been shown to alter Ca 2ϩ sensitivity and/or cross-bridge kinetics, impacting both in vitro and in vivo contractility (31,39). Alterations in the ratio of MHC isoforms and the phosphorylation status of other myofilament proteins have been implicated in the contractile dysfunction characteristic of HF in a variety of animal models (21,25,31,39,40). However, the mechanism(s) by which dietary lipids impact the Ca 2ϩ regulation of contractile function and myofilament protein composition in HF has not been investigated.…”