The Troponin Tail Domain Promotes a Conformational State of the Thin Filament That Suppresses Myosin Activity

Abstract

Calcium binding to the thin filament protein troponin is required for cardiac and skeletal muscles to contract, and several studies indicate that this regulation involves shifting the tropomyosin position on the actin filament. When the regulatory sites of troponin do not have bound Ca 2ϩ , tropomyosin is located on the actin outer domain. In this position tropomyosin sterically interferes with much more of the myosin-binding site than it does in the presence of Ca 2ϩ , and therefore contraction is inhibited at low Ca 2ϩ concentrations. This regulatory scheme is supported by three-dimensional helical reconstructions of thin filaments examined by electron microscopy with negative staining (1, 2) or unstained in vitreous ice (3), and by modeling of x-ray diffraction patterns of muscle (4). Furthermore, it is consistent with the solution kinetics of myosin S1-thin filament binding in the absence of ATP (5). However, it is unclear how troponin affects the position of tropomyosin on actin, and more generally the inhibitory action of troponin is not well understood at a structural level, as opposed to better understandings of tropomyosin.Troponin consists of a relatively globular domain (including TnI 1 and TnC) and an elongated tail region, the NH 2 terminus of TnT (6, 7). The inhibitory actions of troponin have long been attributed to the TnI subunit. Skeletal muscle TnI inhibits the actin-myosin ATPase rate in the absence of the other troponin subunits TnC and TnT (8 -10), and this effect requires lower TnI concentrations in the presence rather than in the absence of tropomyosin (11). Cardiac TnI has similar properties, although the inhibition is less effective (12-14). The inhibitory effects of skeletal muscle TnI can be mimicked by the so-called inhibitory peptide, residues 96 -116 (10, 15), or identically by the corresponding cardiac peptide (14). The reversal of inhibition is related to Ca 2ϩ -dependent TnI-TnC interactions, elucidated in part at the atomic level (16). An additional TnI region, approximately 130 -150 residues, has also been implicated in inhibition (17)(18)(19). These and other data are consistent with an inhibitory mechanism consisting primarily of a TnI-actin interaction that is reversed by Ca 2ϩ , i.e. a localized actin-troponin interaction tethers the much longer tropomyosin on the actin outer domain in the absence, but not in the presence, of Ca 2ϩ . Indeed, our own electron microscope results show that Ca 2ϩ causes a decrease in troponin density in contact with actin (20). However, no high resolution data exists for these interactions, or for the assembled thin filament, and it remains possible that other mechanisms are also important for regulation and for determining the shifting positions of tropomyosin on the actin surface.This report describes new and unexpected attributes of the troponin tail, i.e. the NH 2 terminus of TnT. In the absence of all other portions of troponin, including TnI, cardiac TnT-(1-153) inhibited the interaction of myosin with actin-tropomyosin filaments. Helica...

A Modulatory Role for the Troponin T Tail Domain in Thin Filament Regulation

“…Our results can be compared with those reported for a similar TnT 1 fragment from bovine cardiac troponin by Tobacman et al (44). Their TnT 1 fragment inhibits S1 ATPase, in vitro motility, and S1-ADP binding assays, in agreement with our data.…”

Differential Regulation of the Actomyosin Interaction by Skeletal and Cardiac Troponin Isoforms

“…Recent work has suggested that TnT may in fact have a significant role in modulating regulation, which may contribute to the differences between the two cardiac and skeletal complexes seen here (9,10). Although our study characterized skeletal TnT effects using similar assays to those presented here, the other used motility and ATPase measurements.…”

“…TnT also influences the Tm-Tm communication along the filament via its interaction at the Tm-Tm overlap (8) and has been shown recently to also influence regulation directly. Our own work has shown the TnT 1 fragment of skeletal TnT effects the open-closed equilibrium significantly reducing K T (9), whereas work on a similar cardiac TnT 1 fragment has shown strong inhibition of the actinactivated myosin ATPase (10).…”

F110I and R278C Troponin T Mutations That Cause Familial Hypertrophic Cardiomyopathy Affect Muscle Contraction in Transgenic Mice and Reconstituted Human Cardiac Fibers

“…2 plays many important roles in striated muscle contraction, providing structural stability to the troponin complex and actively participating in the Ca 2ϩ -dependent regulation of contraction (1)(2)(3)(4)(5). TnT was first associated with heart disease in 1994 where three mutations were reported to cause Familial Hypertrophic Cardiomyopathy (FHC) (6).…”