Structural insights into the tropomodulin assembly at the pointed ends of actin filaments

Nucleus Mechanosensing in Cardiomyocytes

Cardiac muscle contraction is distinct from the contraction of other muscle types. The heart continuously undergoes contraction-relaxation cycles throughout an animal's lifespan. It must respond to constantly varying physical and energetic burdens over the short term on a beat-to-beat basis and relies on different mechanisms over the long term. Muscle contractility is based on actin and myosin interactions that are regulated by cytoplasmic calcium ions. Genetic variants of sarcomeric proteins can lead to the pathophysiological development of cardiac dysfunction. The sarcomere is physically connected to other cytoskeletal components. Actin filaments, microtubules and desmin proteins are responsible for these interactions. Therefore, mechanical as well as biochemical signals from sarcomeric contractions are transmitted to and sensed by other parts of the cardiomyocyte, particularly the nucleus which can respond to these stimuli. Proteins anchored to the nuclear envelope display a broad response which remodels the structure of the nucleus. In this review, we examine the central aspects of mechanotransduction in the cardiomyocyte where the transmission of mechanical signals to the nucleus can result in changes in gene expression and nucleus morphology. The correlation of nucleus sensing and dysfunction of sarcomeric proteins may assist the understanding of a wide range of functional responses in the progress of cardiomyopathic diseases.

Effects of Tropomodulin 2 on Dendritic Spine Reorganization and Dynamics

Dendritic spines are actin-rich protrusions that receive a signal from the axon at the synapse. Remodeling of cytoskeletal actin is tightly connected to dendritic spine morphology-mediated synaptic plasticity of the neuron. Remodeling of cytoskeletal actin is required for the formation, development, maturation, and reorganization of dendritic spines. Actin filaments are highly dynamic structures with slow-growing/pointed and fast-growing/barbed ends. Very few studies have been conducted on the role of pointed-end binding proteins in the regulation of dendritic spine morphology. In this study, we evaluated the role played by tropomodulin 2 (Tmod2)-a brain-specific isoform, on the dendritic spine re-organization. Tmod2 regulates actin nucleation and polymerization by binding to the pointed end via actin and tropomyosin (Tpm) binding sites. We studied the effects of Tmod2 overexpression in primary hippocampal neurons on spine morphology using confocal microscopy and image analysis. Tmod2 overexpression decreased the spine number and increased spine length. Destroying Tpm-binding ability increased the number of shaft synapses and thin spine motility. Eliminating the actin-binding abilities of Tmod2 increased the number of mushroom spines. Tpm-mediated pointed-end binding decreased F-actin depolymerization, which may positively affect spine stabilization; the nucleation ability of Tmod2 appeared to increase shaft synapses.

Myofilament-associated proteins with intrinsic disorder (MAPIDs) and their resolution by computational modeling

The cardiac sarcomere is a cellular structure in the heart that enables muscle cells to contract. Dozens of proteins belong to the cardiac sarcomere, which work in tandem to generate force and adapt to demands on cardiac output. Intriguingly, the majority of these proteins have significant intrinsic disorder that contributes to their functions, yet the biophysics of these intrinsically disordered regions (IDRs) have been characterized in limited detail. In this review, we first enumerate these myofilament-associated proteins with intrinsic disorder (MAPIDs) and recent biophysical studies to characterize their IDRs. We secondly summarize the biophysics governing IDR properties and the state-of-the-art in computational tools toward MAPID identification and characterization of their conformation ensembles. We conclude with an overview of future computational approaches toward broadening the understanding of intrinsic disorder in the cardiac sarcomere.

Nuclear Magnetic Resonance-Guided Structural Analysis of Moderate-Affinity Protein Complexes with Intrinsically Disordered Polypeptides

Binding affinity of an individual binding site of an intrinsically disordered protein for its folded partner may be moderate. In such cases, a straightforward determination of the structure of the binding interface is difficult. We offer a hybrid protocol combining NMR chemical shift information, NMR spectral data on amino acid residue sequence substitution effects, residual dipolar coupling, and molecular dynamics simulation that allowed us to determine the structure of a complex between the intrinsically disordered tropomyosin-binding site of leiomodin and a coiled-coil peptide modeling the N-terminal fragment of tropomyosin. The protocol can be used for other moderate-affinity complexes composed of an intrinsically disordered peptide bound to a structured protein partner.

The role of leiomodin in actin dynamics: a new road or a secret gate

Leiomodin is an important emerging regulator of thin filaments. As novel molecular, cellular, animal model, and human data accumulate, the mechanisms of its action become clearer. Structural studies played a significant part in understanding the functional significance of leiomodin's interacting partners and functional domains. In this review, we present the current state of knowledge on the structural and cellular properties of leiomodin which has led to two proposed mechanisms of its function. Although it is known that leiomodin is essential for life, numerous domains within leiomodin remain unstudied and as such, we outline future directions for investigations that we predict will provide evidence that leiomodin is a multifunctional protein.

Ca2+ attenuates nucleation activity of leiomodin

A transient increase in Ca2+ concentration in sarcomeres is essential for their proper function. Ca2+ drives striated muscle contraction via binding to the troponin complex of the thin filament to activate its interaction with the myosin thick filament. In addition to the troponin complex, the myosin essential light chain and myosin-binding protein C were also found to be Ca2+ sensitive. However, the effects of Ca2+ on the function of the tropomodulin family proteins involved in regulating thin filament formation have not yet been studied. Leiomodin, a member of the tropomodulin family, is an actin nucleator and thin filament elongator. Using pyrene-actin polymerization assay and transmission electron microscopy, we show that the actin nucleation activity of leiomodin is attenuated by Ca2+ . Using circular dichroism and nuclear magnetic resonance spectroscopy, we demonstrate that the mostly disordered, negatively charged region of leiomodin located between its first two actin-binding sites binds Ca2+ . We propose that Ca2+ binding to leiomodin results in the attenuation of its nucleation activity. Our data provide further evidence regarding the role of Ca2+ as an ultimate regulator of the ensemble of sarcomeric proteins essential for muscle function. SUMMARY STATEMENT: Ca2+ fluctuations in striated muscle sarcomeres modulate contractile activity via binding to several distinct families of sarcomeric proteins. The effects of Ca2+ on the activity of leiomodin-an actin nucleator and thin filament length regulator-have remained unknown. In this study, we demonstrate that Ca2+ binds directly to leiomodin and attenuates its actin nucleating activity. Our data emphasizes the ultimate role of Ca2+ in the regulation of the sarcomeric protein interactions.

Crystal Structure Analysis and IgE Epitope Mapping of Allergic Predominant Region in Scylla paramamosain Filamin C, Scy p 9

Filamin C (FLN c) is a novel allergen in shellfish. In this study, FLN c from Scylla paramamosain was divided into three regions for recombinant expression based on the number of domains and amino acids. Using dot blot and basophil activation tests, the allergic predominant region of FLN c was determined to be 336-531 amino acid positions (named FLN c-M). It was confirmed that by X-ray diffraction, the crystal structure of FLN c-M with immunoglobulin-like folding at a resolution of 1.7 Å was obtained. The monomer was a barrel structure composed of 16 β-strands and 2 α-helices. Three conformational epitopes were predicted, six linear epitopes were verified by serological test, and they were positioned on the crystal structure of FLN c-M. For the first time, the crystal structure of the allergic predominant region of FLN c was determined, and it provided an accurate template for the localization of IgE epitopes.