A solution to the long-standing problem of actin expression and purification

Structural dynamics of the intrinsically disordered linker region of cardiac troponin T

The cardiac troponin complex, composed of troponins I, T, and C, plays a central role in regulating the calcium-dependent interactions between myosin and the thin filament. Mutations in troponin can cause cardiomyopathies; however, it is still a major challenge for the field to connect how changes in sequence affect troponin's function. Recent high-resolution structures of the thin filament revealed critical insights into the structure-function relationship of the troponin complex, but there remain large, unresolved segments of troponin, including the troponin-T linker region that is a hotspot for several cardiomyopathy mutations. This unresolved yet functionally-significant linker region has been proposed to be intrinsically disordered, with behaviors that are not well described by traditional structural approaches; however, this proposal has not been experimentally verified. Here, we used a combination of single-molecule Förster resonance energy transfer (FRET), molecular dynamics simulations, and functional reconstitution assays to investigate the troponin-T linker region. We experimentally and computationally show that in the context of both isolated troponin and the fully regulated troponin complex, the linker behaves as a dynamic, intrinsically disordered region. This region undergoes polyampholyte expansion in the presence of high salt and distinct conformational changes during the assembly of the troponin complex. We also examine the ΔE160 hypertrophic cardiomyopathy mutation in the linker, and we demonstrate that this mutation does not affect the conformational dynamics of the linker, rather it allosterically affects interactions with other subunits of the troponin complex, leading to increased molecular contractility. Taken together, our data clearly demonstrate the importance of disorder within the troponin-T linker and provide new insights into the molecular mechanisms controlling the pathogenesis of cardiomyopathies.

Molecular mechanisms linking missense ACTG2 mutations to visceral myopathy

Visceral myopathy is a life-threatening disease characterized by muscle weakness in the bowel, bladder, and uterus. Mutations in smooth muscle γ-actin (ACTG2) are the most common cause of the disease, but the mechanisms by which the mutations alter muscle function are unknown. Here, we examined four prevalent ACTG2 mutations (R40C, R148C, R178C, and R257C) that cause different disease severity and are spread throughout the actin fold. R178C displayed premature degradation, R148C disrupted interactions with actin-binding proteins, R40C inhibited polymerization, and R257C destabilized filaments. Because these mutations are heterozygous, we also analyzed 50/50 mixtures with wild-type (WT) ACTG2. The WT/R40C mixture impaired filament nucleation by leiomodin 1, and WT/R257C produced filaments that were easily fragmented by smooth muscle myosin. Smooth muscle tropomyosin isoform Tpm1.4 partially rescued the defects of R40C and R257C. Cryo-electron microscopy structures of filaments formed by R40C and R257C revealed disrupted intersubunit contacts. The biochemical and structural properties of the mutants correlate with their genotype-specific disease severity.

Understanding the Role of Filamentous Actin in Food Quality: From Structure to Application

Actin, a multifunctional protein highly expressed in eukaryotes, is widely distributed throughout cells and serves as a crucial component of the cytoskeleton. Its presence is integral to maintaining cell morphology and participating in various biological processes. As an irreplaceable component of myofibrillar proteins, actin, including G-actin and F-actin, is highly related to food quality. Up to now, purification of actin at a moderate level remains to be overcome. In this paper, we have reviewed the structures and functions of actin, the methods to obtain actin, and the relationships between actin and food texture, color, and flavor. Moreover, actin finds applications in diverse fields such as food safety, bioengineering, and nanomaterials. Developing an actin preparation method at the industrial level will help promote its further applications in food science, nutrition, and safety.

Structural and functional mechanisms of actin isoforms

Actin is a highly conserved and fundamental protein in eukaryotes and participates in a broad spectrum of cellular functions. Cells maintain a conserved ratio of actin isoforms, with muscle and non-muscle actins representing the main actin isoforms in muscle and non-muscle cells, respectively. Actin isoforms have specific and redundant functional roles and display different biochemistries, cellular localization, and interactions with myosins and actin-binding proteins. Understanding the specific roles of actin isoforms from the structural and functional perspective is crucial for elucidating the intricacies of cytoskeletal dynamics and regulation and their implications in health and disease. Here, we review how the structure contributes to the functional mechanisms of actin isoforms with a special emphasis on the questions of how post-translational modifications and disease-linked mutations affect actin isoforms biochemistry, function, and interaction with actin-binding proteins and myosin motors.

Biochemical characterization of cardiac α-actin mutations A21V and D26N implicated in hypertrophic cardiomyopathy

Familial hypertrophic cardiomyopathy (HCM) affects .2% of the world's population and is inherited in an autosomal dominant manner. Mutations in cardiac α-actin are the cause in 1%-5% of all observed cases. Here, we describe the recombinant production, purification, and characterization of the HCM-linked cardiac α-actin variants p.A21V and p.D26N. Mass spectrometric analysis of the initially purified recombinant cardiac α-actin variants and wild-type protein revealed improper N-terminal processing in the Spodoptera frugiperda (Sf-9) insect cell system, compromising the labeling of the protein with fluorescent probes for biochemical studies. Therefore, we produced N-terminal deletion mutants lacking the N-terminal cysteine (ΔC2). The ΔC2 wild-type construct behaved similar to porcine cardiac α-actin purified from native Sus scrofa heart tissue and all ΔC2 constructs showed improved fluorescent labeling. Further analysis of untruncated and ΔC2 constructs showed that while neither the A21V nor the D26N mutation affects nucleotide binding, they cause a similar slowing of the rate of filament formation as well as a reduction in the thermal stability of monomeric and filamentous cardiac α-actin. In vitro motility assays and transient-kinetic studies probing the interaction of the actin variants with cardiac β-myosin revealed perturbed actomyosin interactions and a reduced motile activity for the p.D26N variant. Addition of the small molecule effector EMD 57033, which targets cardiac β-myosin, rescued the approximately 40% drop in velocity observed with the p.D26N constructs and activated the motile activity of wild-type and p.D26N to the same level of 1100 nm s-1 .

Purification of Human Cytoplasmic Actins From Saccharomyces cerevisiae

Eukaryotic cells rely on actin to support cellular structure, motility, transport, and a wide variety of other cytoplasmic functions and nuclear activities. Humans and other mammals express six closely related isoforms of actin, four of which are found primarily in skeletal, cardiac, and smooth muscle tissues. The final two isoforms, β and γ, are found in non-muscle cells. Due to the ease of purification, many biochemical studies surveying the functions of actin and its regulators have been carried out with protein purified from skeletal muscle. However, it has become increasingly clear that some activities are isoform specific, necessitating more accessible sources of non-muscle actin isoforms. Recent innovations permit the purification of non-muscle actins from human cell culture and heterologous systems, such as insect cell culture and the yeast Pichia pastoris. However, these systems generate mixtures of actin types or require additional steps to remove purification-related tags. We have developed strains of Saccharomyces cerevisiae (budding yeast) that express single untagged isoforms of either human non-muscle actin (β or γ) as their sole actin, allowing the purification of individual homogeneous actin isoforms by conventional purification techniques. Key features • Easy growth of yeast as a source of human cytoplasmic actin isoforms. Uses well-established actin purification methods. • The tag-free system requires no post-purification processing.

Purification of modified mammalian actin isoforms for in vitro reconstitution assays

In vitro reconstitution assays using purified actin have greatly improved our understanding of cytoskeletal dynamics and their regulation by actin-binding proteins. However, early purification methods consisted of harsh conditions to obtain pure actin and often did not include correct maturation and obligate modification of the isolated actin monomers. Novel insights into the folding requirements and N-terminal processing of actin as well as a better understanding of the interaction of actin with monomer sequestering proteins such as DNaseI, profilin and gelsolin, led to the development of more gentle approaches to obtain pure recombinant actin isoforms with known obligate modifications. This review summarizes the approaches that can be employed to isolate natively folded endogenous and recombinant actin from tissues and cells. We further emphasize the use and limitations of each method and describe how these methods can be implemented to study actin PTMs, disease-related actin mutations and novel actin-like proteins.

The actin cytoskeleton in hair bundle development and hearing loss

Inner ear hair cells assemble mechanosensitive hair bundles on their apical surface that transduce sounds and accelerations. Each hair bundle is comprised of ∼ 100 individual stereocilia that are arranged into rows of increasing height and width; their specific and precise architecture being necessary for mechanoelectrical transduction (MET). The actin cytoskeleton is fundamental to establishing this architecture, not only by forming the structural scaffold shaping each stereocilium, but also by composing rootlets and the cuticular plate that together provide a stable foundation supporting each stereocilium. In concert with the actin cytoskeleton, a large assortment of actin-binding proteins (ABPs) function to cross-link actin filaments into specific topologies, as well as control actin filament growth, severing, and capping. These processes are individually critical for sensory transduction and are all disrupted in hereditary forms of human hearing loss. In this review, we provide an overview of actin-based structures in the hair bundle and the molecules contributing to their assembly and functional properties. We also highlight recent advances in mechanisms driving stereocilia elongation and how these processes are tuned by MET.

Purification of human β- and γ-actin from budding yeast

Biochemical studies of human actin and its binding partners rely heavily on abundant and easily purified α-actin from skeletal muscle. Therefore, muscle actin has been used to evaluate and determine the activities of most actin regulatory proteins but there is an underlying concern that these proteins perform differently from actin present in non-muscle cells. To provide easily accessible and relatively abundant sources of human β- or γ-actin (i.e. cytoplasmic actins), we developed Saccharomyces cerevisiae strains that express each as their sole source of actin. Both β- or γ-actin purified in this system polymerize and interact with various binding partners, including profilin, mDia1 (formin), fascin and thymosin-β4 (Tβ4). Notably, Tβ4 and profilin bind to β- or γ-actin with higher affinity than to α-actin, emphasizing the value of testing actin ligands with specific actin isoforms. These reagents will make specific isoforms of actin more accessible for future studies on actin regulation.