Residual force enhancement (RFE) is a property of skeletal muscle where more force is produced after an active stretch than if it were simply activated at the longer length. This property is important for jumping, locomotive, and stabilizing movements of the body. The molecular mechanism underlying RFE is not well understood.

In muscle, the protein titin connects myofilaments and has been shown to have many roles in sarcomere stability and modulating contraction. The authors of this study investigated titin’s function during contraction using small-angle X-ray diffraction of untreated WT mouse skeletal muscle and muscle where 50 …

Understanding the driving forces behind stability of denatured state ensembles (DSE’s) and intrinsically disordered proteins (IDP’s) is central to a number of unresolved questions in bimolecular thermodynamics regarding protein folding pathways and foldability, thermodynamic stability, aggregation and misfolding. Researchers at the University of Chicago and Notre Dame used temperature-controlled size-exclusion chromatography-coupled SAXS (SEC-SAXS) and NMR to examine how temperature and solvent ionic strength influences the solution structure(s) of the N-terminal domain of pertactin (PNt). PNt is a valuable model system from a fundamental biophysical point of view, as the full-length 539-residue pertactin folds into a parallel β-helix but the 334 N-terminal residues do not and instead behave as a highly expanded, intrinsically disordered chain.

In this work, the authors used temperature-series SEC-SAXS experiments performed at BioCAT, with temperatures ranging from 4 to 60 degrees C, to show that PNt displays a mild, temperature-dependent contraction (as measured by R_g value). Glycine-rich low hydrophobicity model systems (as proxies for protein backbones alone) did not display an analogous trend, showing the specific importance of side chain interactions. Increasing the ionic strength of the …

The Frank-Starling Law of the Heart states that the heart’s stroke volume increases with greater preload due to increased venous return, allowing the heart to adapt to varying circulatory demands. At the molecular level, increasing preload increases sarcomere length (SL), which alters structures within the sarcomere that are correlated to increased calcium sensitivity upon activation. The titin protein, spanning the half-sarcomere acts as a spring in the I-band, applies a SL-dependent passive force on the myosin containing thick filaments changing its structure and functional properties. Altered titin-based forces play a crucial role in the etiology of many cardiomyopathies; however, the disease state obscures titin’s role, impeding therapeutic solutions.

The authors studied titin’s specific role using the titin cleavage (TC) mouse model, where a tobacco-etch virus protease (TEVP) recognition site is inserted into distal I-band titin and allows for rapid, specific cleavage …

Muscle contraction is produced via the interaction of myofilaments and is regulated so that muscle performance matches demand. Myosin-binding protein C (MyBP-C) is a long and flexible protein that is tightly bound to the thick filament at its C-terminal end (MyBP-C^C8C10), but may be loosely bound at its middle- and N-terminal end (MyBP-C^C1C7) to myosin heads and/or the thin filament. MyBP-C is thought to control muscle contraction via the regulation of myosin motors, as mutations lead to debilitating disease. Thick filament based regulatory mechanisms involve either sequestering myosin head into an inactive ordered off state close to the thick filament backbone or releasing them so that they can interact with actin. Strain in the thick filaments, either from a few active heads or passively generated by stretching titin, is presumed to act as a trigger to release inactive heads. While MyBP-C has been presumed to play a role(s) in thick filament activation, details of these mechanisms have been lacking. Here the authors used combination of mechanics and small-angle X-ray diffraction to study the effects of immediate and selective removal of the MyBP-C^C1C7 domains of fast MyBP-C on sarcomere structure and function in permeabilized skeletal muscle …

Netrins are a conserved class of proteins involved in synaptic connectivity of the nervous system in bilaterian animals. They act as secreted guidance cues, with the unique ability to exert repulsive and attractive responses on growing axons. They are also known to be involved in cell proliferation, migration and differentiation, and are therefore targets for treating cancer and insulin resistance. During axon growth and cell migration, the presence of the receptor Uncoordinated-5 (UNC-5) on target cells results in repulsion. However, the exact mechanism involved in the induction of repulsive forces has been relatively unknown due to the lack of biochemical and structural information about these systems. Researchers at the University of Chicago and Stanford University, in collaboration with BioCAT staff, showed that UNC-5 is a heparin-binding protein, determined its structure bound to a heparin fragment, and could modulate UNC-5–heparin affinity using a directed evolution platform or structure-based rational design.

A question they needed to answer was whether the heparin + UNC-6–mediated UNC-5 oligomers are an ordered and well-defined molecular species or (ii) whether these oligomers …

Mavacamten is the first myosin-targeted small-molecule therapy approved by the Food and Drug Administration to treat obstructive hypertrophic cardiomyopathy by attenuating excessive myocardial sarcomere activity. Mavacamten regulates cardiac function at the sarcomere level by selectively but reversibly inhibiting the enzymatic activity of myosin. It shifts myosin toward ordered off states close to the thick filament backbone making them unavailable for binding to actin and generating force. It is necessary, however for the heart to adjust its output to ensure sufficient cardiac output, especially during increased physiological demands. It was unknown whether mavacamten stabilized heads could still be recruited by the usual physiological inotropic mechanisms for the patient to be able to adapt to changing demands on their hearts …

Myosin-based thick-filament regulation is now known to be critical for muscle contraction with myosin dysregulation found in hypertrophic and dilated cardiomyopathies but many details of thick filament regulation remain to be discovered. Myosin ATPase assays have demonstrated that under relaxed conditions, myosin may reside either in a high-energy-consuming disordered-relaxed (DRX) state available for binding actin to generate force or in an energy-sparing super-relaxed (SRX) state unavailable for actin binding. X-ray diffraction studies have shown that the majority of myosin heads are in a …