Impact of A134 and E218 Amino Acid Residues of Tropomyosin on Its Flexibility and Function

Glutamate 139 of tropomyosin is critical for cardiac thin filament blocked-state stabilization

The cardiac thin filament proteins troponin and tropomyosin control actomyosin formation and thus cardiac contractility. Calcium binding to troponin changes tropomyosin position along the thin filament, allowing myosin head binding to actin required for heart muscle contraction. The thin filament regulatory proteins are hot spots for genetic mutations causing heart muscle dysfunction. While much of the thin filament structure has been characterized, critical regions of troponin and tropomyosin involved in triggering conformational changes remain unresolved. A poorly resolved region, helix-4 (H4) of troponin I, is thought to stabilize tropomyosin in a position on actin that blocks actomyosin interactions at low calcium concentrations during muscle relaxation. We have proposed that contact between glutamate 139 on tropomyosin and positively charged residues on H4 leads to blocking-state stabilization. In this study, we attempted to disrupt these interactions by replacing E139 with lysine (E139K) to define the importance of this residue in thin filament regulation. Comparison of mutant and wild-type tropomyosin was carried out using in-vitro motility assays, actin co-sedimentation, and molecular dynamics simulations to determine perturbations in troponin-tropomyosin function caused by the tropomyosin mutation. Motility assays revealed that mutant thin filaments moved at higher velocity at low calcium with increased calcium sensitivity demonstrating that tropomyosin residue 139 is vital for proper tropomyosin-mediated inhibition during relaxation. Similarly, molecular dynamic simulations revealed a mutation-induced decrease in interaction energy between tropomyosin-E139K and troponin I (R170 and K174). These results suggest that salt-bridge stabilization of tropomyosin position by troponin IH4 is essential to prevent actomyosin interactions during cardiac muscle relaxation.

Novel Mutation Glu98Lys in Cardiac Tropomyosin Alters Its Structure and Impairs Myocardial Relaxation

We characterized a novel genetic variant c.292G > A (p.E98K) in the TPM1 gene encoding cardiac tropomyosin 1.1 isoform (Tpm1.1), found in a proband with a phenotype of complex cardiomyopathy with conduction dysfunction and slow progressive neuromuscular involvement. To understand the molecular mechanism by which this mutation impairs cardiac function, we produced recombinant Tpm1.1 carrying an E98K substitution and studied how this substitution affects the structure of the Tpm1.1 molecule and its functional properties. The results showed that the E98K substitution in the N-terminal part of the Tpm molecule significantly destabilizes the C-terminal part of Tpm, thus indicating a long-distance destabilizing effect of the substitution on the Tpm coiled-coil structure. The E98K substitution did not noticeably affect Tpm's affinity for F-actin but significantly impaired Tpm's regulatory properties. It increased the Ca2+ sensitivity of the sliding velocity of regulated thin filaments over cardiac myosin in an in vitro motility assay and caused an incomplete block of the thin filament sliding at low Ca2+ concentrations. The incomplete motility block in the absence of Ca2+ can be explained by the loosening of the Tpm interaction with troponin I (TnI), thus increasing Tpm mobility on the surface of an actin filament that partially unlocks the myosin binding sites. This hypothesis is supported by the molecular dynamics (MD) simulation that showed that the E98 Tpm residue is involved in hydrogen bonding with the C-terminal part of TnI. Thus, the results allowed us to explain the mechanism by which the E98K Tpm mutation impairs sarcomeric function and myocardial relaxation.

De Novo Asp219Val Mutation in Cardiac Tropomyosin Associated with Hypertrophic Cardiomyopathy

Hypertrophic cardiomyopathy (HCM), caused by mutations in thin filament proteins, manifests as moderate cardiac hypertrophy and is associated with sudden cardiac death (SCD). We identified a new de novo variant, c.656A>T (p.D219V), in the TPM1 gene encoding cardiac tropomyosin 1.1 (Tpm) in a young SCD victim with post-mortem-diagnosed HCM. We produced recombinant D219V Tpm1.1 and studied its structural and functional properties using various biochemical and biophysical methods. The D219V mutation did not affect the Tpm affinity for F-actin but increased the thermal stability of the Tpm molecule and Tpm-F-actin complex. The D219V mutation significantly increased the Ca2+ sensitivity of the sliding velocity of thin filaments over cardiac myosin in an in vitro motility assay and impaired the inhibition of the filament sliding at low Ca2+ concentration. The molecular dynamics (MD) simulation provided insight into a possible molecular mechanism of the effect of the mutation that is most likely a cause of the weakening of the Tpm interaction with actin in the "closed" state and so makes it an easier transition to the “open” state. The changes in the Ca2+ regulation of the actin-myosin interaction characteristic of genetic HCM suggest that the mutation is likely pathogenic.