Examination into protein assembly within the cardiac thin filament: Why the troponin complex crosses over.

Abstract

The major contributor to overall function of the cardiac thin filament (cTF) originates from the presence of the troponin (Tn) complex. Despite its clear importance to the function of the thin filament complex, it proves a challenging complex to study due to it containing highly flexible and unstructured regions. Recent cryo-electon microscopy studies have highlighted the organization of the Tn complex within the thin filament, revealing that the troponin complex “crosses-over” the thin filament. The alpha helical portion of TnT interacting with Tm dimers on one side of the thin filament, while its C-terminal end, with the remainder of the Tn core, interacts with the opposing Tm dimer on the other side. With the additional observation that the Tn core interacts primarily with pseudo-repeat 4 of Tm, this results in a heterogeneous organization of Tn within the thin filament through the flexible linker of TnT. To elucidate why Tn organizes in such a fashion, we will utilize our atomic model for the cardiac thin filament with molecular dynamics simulations to observe the overall structure and energetics when the linker crosses over the thin filament and a scenario when it does not; therefore, Tn interacts with the Tm dimers on the same side. Additionally, we will examine Tn-Tm interactions when the Tn core interacts with the other pseudo-repeats of Tm. In these hypothetical thin filament arrangements, we hope to categorize the residue-residue constants and energetics to explain the preferential thin filament assembly observed experimentally.