Difference in polymerization and steady-state dynamics of free and gelsolin-capped filaments formed by α- and β-isoactins

Actin polymerization and depolymerization in developing vertebrates

Development is a complex process that occurs throughout the life cycle. F-actin, a major component of the cytoskeleton, is essential for the morphogenesis of tissues and organs during development. F-actin is formed by the polymerization of G-actin, and the dynamic balance of polymerization and depolymerization ensures proper cellular function. Disruption of this balance results in various abnormalities and defects or even embryonic lethality. Here, we reviewed recent findings on the structure of G-actin and F-actin and the polymerization of G-actin to F-actin. We also focused on the functions of actin isoforms and the underlying mechanisms of actin polymerization/depolymerization in cellular and organic morphogenesis during development. This information will extend our understanding of the role of actin polymerization in the physiologic or pathologic processes during development and may open new avenues for developing therapeutics for embryonic developmental abnormalities or tissue regeneration.

The remodelling of actin composition as a hallmark of cancer

Actin is a key structural protein that makes up the cytoskeleton of cells, and plays a role in functions such as division, migration, and vesicle trafficking. It comprises six different cell-type specific isoforms: ACTA1, ACTA2, ACTB, ACTC1, ACTG1, and ACTG2. Abnormal actin isoform expression has been reported in many cancers, which led us to hypothesize that it may serve as an early biomarker of cancer. We show an overview of the different actin isoforms and highlight mechanisms by which they may contribute to tumorigenicity. Furthermore, we suggest how the aberrant expression of actin subunits can confer cells with greater proliferation ability, increased migratory capability, and chemoresistance through incorporation into the normal cellular F-actin network and altered actin binding protein interaction. Studying this fundamental change that takes place within cancer cells can further our understanding of neoplastic transformation in multiple tissue types, which can ultimately aid in the early-detection, diagnosis and treatment of cancer.

Cooperative effects of tropomyosin on the dynamics of the actin filament

Tropomyosin (Tpm) plays an important role in regulating the organisation and functions of the actin cytoskeleton. Here, we describe a new approach to analyse the effects of Tpm on actin dynamics. Using F-actin proteolytically modified within the DNase-binding loop (ECP-actin), we show that Tpm binding almost completely suppresses the increased subunit exchange intrinsic for this F-actin. The effect is both concentration-dependent and cooperative, with half-maximal inhibition observed at about a 1 : 50 Tpm : actin ratio. Tpm decreases not only the number concentration of ECP-actin filaments, but also the rate of the filament subunit exchange. Our data suggest that Tpm regulates the dynamics of actin filaments by an allosteric strengthening of intermonomer contacts in the actin filament, and that this mechanism may be involved in the modulation of cytoskeletal dynamics.

Actin isoforms and reorganization of adhesion junctions in epithelial-to-mesenchymal transition of cervical carcinoma cells

Malignant cell transformation requires changes in the ability of cells to migrate. The disruption of actin cytoskeleton and intercellular adhesions is an important component of the acquisition of invasive properties in epithelial malignancies. The invasive ability of carcinoma cells is associated with reduced expression of adhesion junction molecules and increased expression of mesenchymal markers, frequently referred to as epithelial-to-mesenchymal transition (EMT). Standard features of the EMT program in cancer cells include fibroblastic phenotype, downregulation of the epithelial marker E-cadherin, induction of Snail-family transcription factors, as well as expression of mesenchymal proteins. We compared the epithelial and mesenchymal marker profiles of nonmalignant HaCaT keratinocytes to the corresponding profiles of cervical carcinoma cell lines C-33A, SiHa, and CaSki. The characteristics of the EMT appeared to be more developed in SiHa and CaSki cervical cancer cells. Further activation of the EMT program in cancer cells was induced by epidermal growth factor. Decreased epithelial marker E-cadherin in CaSki cells was accompanied by increased mesenchymal markers N-cadherin and vimentin. Downregulated expression of E-cadherin in SiHa and CaSki cells was associated with increased expression of Snail transcription factor. Our goal was to study actin reorganization in the EMT process in cell cultures and in tissue. We found that β-cytoplasmic actin structures are disorganized in the cervical cancer cells. The expression of β-cytoplasmic actin was downregulated.

Actin isoform-specific conformational differences observed with hydrogen/deuterium exchange and mass spectrometry

Actin can exist in multiple conformations necessary for normal function. Actin isoforms, although highly conserved in sequence, exhibit different biochemical properties and cellular roles. We used amide proton hydrogen/deuterium (HD) exchange detected by mass spectrometry to analyze conformational differences between Saccharomyces cerevisiae and muscle actins in the G and F forms to gain insight into these differences. We also utilized HD exchange to study interdomain and allosteric communication in yeast-muscle hybrid actins to better understand the conformational dynamics of actin. Areas showing differences in HD exchange between G- and F-actins are areas of intermonomer contacts, consistent with the current filament models. Our results showed greater exchange for yeast G-actin compared with muscle actin in the barbed end pivot region and areas in subdomains 1 and 2 and for F-actin in monomer-monomer contact areas. These results suggest greater flexibility of the yeast actin monomer and filament compared with muscle actin. For hybrid G-actins, the muscle-like and yeastlike parts of the molecule generally showed exchange characteristics resembling their parent actins. A few exceptions were a peptide on top of subdomain 2 and the pivot region between subdomains 1 and 3 with muscle actin-like exchange characteristics although the areas were yeastlike. These results demonstrate that there is cross-talk between subdomains 1 and 2 and the large and small domains. Hybrid F-actin data showing greater exchange compared with both yeast and muscle actins are consistent with mismatched yeast-muscle interfaces resulting in decreased stability of the hybrid filament contacts.