9MO8

Structure of native murine cardiac thin filament at pCa=5.8 in Ca2+-free state (upper strand)

Summary for 9MO8

• Entry DOI: 10.2210/pdb9mo8/pdb
• EMDB information: 48451
• Descriptor: Actin, alpha cardiac muscle 1, Troponin C, slow skeletal and cardiac muscles, Troponin I, cardiac muscle, ... (7 entities in total)
• Functional Keywords: thin filament, cryo-em, troponin, tropomyosin, muscle structure, motor protein
• Biological source: Mus musculus (house mouse); More
• Total number of polymer chains: 15
• Total formula weight: 540536.38

Authors

Risi, C.M.,Galkin, V.E. (deposition date: 2024-12-25, release date: 2025-05-28, Last modification date: 2025-07-02)

Primary citation

Risi, C.M.,Landim-Vieira, M.,Belknap, B.,Chase, P.B.,Pinto, J.R.,Galkin, V.E.
The role of the troponin T interactions with actin in regulation of cardiac thin filament revealed by the troponin T pathogenic variant Ile79Asn.
J.Mol.Cell.Cardiol., 204:55-67, 2025

PubMed Abstract: Cardiac muscle contraction/relaxation cycle depends on the rising and falling Ca levels in sarcomeres that control the extent of interactions between myosin-based thick and actin-based thin filaments. Cardiac thin filament (cTF) consists of actin, tropomyosin (Tm) that regulates myosin binding to actin, and troponin complex that governs Tm position upon Ca-binding. Troponin has three subunits - Ca-binding troponin C (TnC), Tm stabilizing troponin T (TnT), and inhibitory troponin I (TnI). TnT N-terminus (TnT1) interactions with actin stabilize the inhibited state of cTF. TnC, TnI, and Tm work in concert to control actomyosin interactions. Cryo-electron microscopy (cryo-EM) provided factual structures of healthy cTF, but structures of cTF carrying missense mutations linked to human cardiomyopathy are unknown. Variant Ile79Asn in human cardiac TnT (TnT-I79N) increases myofilament Ca sensitivity and slows cross-bridge kinetics, leading to severe hypertrophic/restrictive cardiomyopathy. Here, we used TnT-I79N mutation as a tool to examine the role of TnT1 in the complex mechanism of cTF regulation. Comparison of the cryo-EM structures of murine wild type and TnT-I79N native cTFs at systolic Ca levels (pCa = 5.8) demonstrates that TnT-I79N causes 1) dissociation of the TnT1 loop from its actin interface that results in Tm release to a more activated position, 2) reduced interaction of TnI C-terminus with actin-Tm, and 3) increased frequency of Ca-bound regulatory units. Our data indicate that the TnT1 loop is a crucial element of the allosteric regulatory network that couples Tn subunits and Tm to maintain adequate cTF response to physiological Ca levels during a heartbeat.

PubMed: 40412797
DOI: 10.1016/j.yjmcc.2025.05.005

Experimental method

ELECTRON MICROSCOPY (5.2 Å)