Structure of the core domain of human cardiac troponin in the Ca2+-saturated form

Abstract

Troponin is essential in Ca(2+) regulation of skeletal and cardiac muscle contraction. It consists of three subunits (TnT, TnC and TnI) and, together with tropomyosin, is located on the actin filament. Here we present crystal structures of the core domains (relative molecular mass of 46,000 and 52,000) of human cardiac troponin in the Ca(2+)-saturated form. Analysis of the four-molecule structures reveals that the core domain is further divided into structurally distinct subdomains that are connected by flexible linkers, making the entire molecule highly flexible. The alpha-helical coiled-coil formed between TnT and TnI is integrated in a rigid and asymmetric structure (about 80 angstrom long), the IT arm, which bridges putative tropomyosin-anchoring regions. The structures of the troponin ternary complex imply that Ca(2+) binding to the regulatory site of TnC removes the carboxy-terminal portion of TnI from actin, thereby altering the mobility and/or flexibility of troponin and tropomyosin on the actin filament.

Invited review: Probing the structures of muscle regulatory proteins using small‐angle solution scattering

“…Although the specific structural features of this extension cannot be deduced from the low-resolution neutron models, the novel finding of this structural change in sTnI has laid a foundation for further high-resolution studies. 15,[63][64][65] All optimized SANS-derived models point to a similar shape for sTnC, that of a fully extended dumbbell shape in both Ca 21 -free and Ca 21 -bound states, consistent with previous interpretations 48,49 as opposed to the somewhat more compact conformation observed in the cTn crystal structure 60 and SANS models for cTn. 50,51 In the SANS-derived model for sTn, the N-terminal domain of Ca 21 -bound sTnC rotates more than 1008 away from the position seen in the crystal structure.…”

“…The crystal structure of the core domain of human cardiac troponin complex 3Ca 21 Á cTnC-cTnI(31-210)-cTnT(183-288) by Takeda et al 60 was an important breakthrough in the troponin field. Although the relative dispositions of individual subdomains in the crystal structure, including the somewhat compact cTnC, were found to be consistent with the earlier SANS-derived model, 51 this crystal structure provided the opportunity for the first high-resolution structural interpretation of the Ca 21 -regulatory mechanism.…”

“…Differences are also evident because of the known inherent flexibility in cTn from the crystal structures of the cTn core that has high B-factors and ambiguous density regions and also substantial differences between two crystal forms. 60 The SANS-derived model represents the ensemble average of the solution form.…”

A preferred AMPK phosphorylation site adjacent to the inhibitory loop of cardiac and skeletal troponin I

“…7). Ser149, a highly conserved residue in TnI, is located adjacent to the inhibitory loop, a segment spanning amino acids 136-147 (human sequence excluding the initial methionine) that has been shown to be the minimum sequence of cTnI that prevents actomyosin ATPase activity in the absence of Ca 2þ -bound cTnC [46][47][48] and therefore, key for the inhibitory properties of TnI. Ser149 is also located adjacent to the switch region (amino acids 150-159), an amphiphilic a-helix that binds a hydrophobic cleft on TnC and is thought to act as a Ca 2þ transducer, signaling the binding of Ca 2þ to TnC to the rest of the contractile apparatus.…”

“…Ser149 is also located adjacent to the switch region (amino acids 150-159), an amphiphilic a-helix that binds a hydrophobic cleft on TnC and is thought to act as a Ca 2þ transducer, signaling the binding of Ca 2þ to TnC to the rest of the contractile apparatus. [46][47][48] Therefore, Ser149 is strategically positioned to possibly influence the inhibitory properties of TnI toward actin-myosin interactions and the affinity of TnI for TnC.…”