Structure of the 34 kDa F-actin-bundling protein ABP34 from Dictyostelium discoideum

The Dictyostelium discoideum FimA protein, unlike yeast and plant fimbrins, is regulated by calcium similar to mammalian plastins

Plastins, also known as fimbrins, are highly conserved eukaryotic multidomain proteins that are involved in actin-bundling. They all contain four independently folded Calponin Homology-domains and an N-terminal headpiece that is comprised of two calcium-binding EF-hand motifs. Since calcium-binding has been shown to be integral to regulating the activity of the three mammalian plastin proteins, we decided to study the properties of the headpiece regions of fimbrins from the model plant Arabidopsis thaliana, the yeasts Saccharomyces cerevisiae and Schizosaccharomyces pombe and the amoeba Dictyostelium discoideum. Of these protein domains only the FimA headpiece from the amoeba protein possesses calcium binding properties. Structural characterization of this protein domain by multidimensional NMR and site-directed mutagenesis studies indicates that this EF-hand region of FimA also contains a regulatory 'switch helix' that is essential to regulating the activity of the human L-plastin protein. Interestingly this regulatory helical region seems to be lacking in the plant and yeast proteins and in fimbrins from all other nonmotile systems. Typical calmodulin antagonists can displace the switch-helix from the FimA headpiece, suggesting that such drugs can deregulate the Ca2+-regulation of the actin-bunding in the amoeba, thereby making it a useful organism for drug screening against mammalian plastins.

Microscopic studies on severing properties of actin-binding protein: its potential use in therapeutic treatment of actin-rich inclusions

Actin is an important unit of the cytoskeletal system, involved in many cellular processes including cell motility, signaling, and intracellular trafficking. Various studies have been undertaken to understand the regulatory mechanisms pertaining actin functions, especially the ones controlled by actin-binding proteins. However, not much has been explored about the molecular aspects of these proteins implicated in various diseases. In this study, we aimed to demonstrate the molecular properties of gelsolin, an actin-severing protein on the disassembly of the aggregation of actin-rich intracellular inclusions, Hirano body. We observed a decreasing tendency of actin aggregation by co-sedimentation assay and transmission electron microscopy in the presence of gelsolin. Therefore, we provide suggestive evidence for the use of actin-severing protein in novel therapeutic strategies for neurodegenerative conditions.

The actin cytoskeleton orchestra in Entamoeba histolytica

Years of evolution have kept actin conserved throughout various clades of life. It is an essential protein starring in many cellular processes. In a primitive eukaryote named Entamoeba histolytica, actin directs the process of phagocytosis. A finely tuned coordination between various actin-binding proteins (ABPs) choreographs this process and forms one of the virulence factors for this protist pathogen. The ever-expanding world of ABPs always has space to accommodate new and varied types of proteins to the earlier existing repertoire. In this article, we report the identification of 390 ABPs from Entamoeba histolytica. These proteins are part of diverse families that have been known to regulate actin dynamics. Most of the proteins are primarily uncharacterized in this organism; however, this study aims to annotate the ABPs based on their domain arrangements. A unique characteristic about some of the ABPs found is the combination of domains present in them unlike any other reported till date. Calponin domain-containing proteins formed the largest group among all types with 38 proteins, followed by 29 proteins with the infamous BAR domain in them, and 23 proteins belonging to actin-related proteins. The other protein families had a lesser number of members. Presence of exclusive domain arrangements in these proteins could guide us to yet unknown actin regulatory mechanisms prevalent in nature. This article is the first step to unraveling them.

The actin bundling activity of actin bundling protein 34 is inhibited by calcium binding to the EF2

Actin bundling protein 34 (ABP34) is the one of 11 actin-crosslinking proteins identified in Dictyostelium discoideum, a novel model organism for the study of actin-associated neurodegenerative disorders such as Alzheimer's disease and Huntington's disease. ABP34 localizes at the leading and trailing edges of locomotory cells, i.e., at the cell cortex, filopodia, and pseudopodia. Functionally, it serves to stabilize membrane-associated actin at sites of cell-cell contact. In addition, this small crosslinking protein is involved in actin bundle formation, and its bundling activity is regulated by the concentration of calcium ion. Several studies have sought to determine the mechanism underlying the calcium-regulated actin bundling activity of ABP34, but it remains unclear. Using several mutational and structural analyses, we revealed that calcium binding to the EF2 motif disrupts the inter-domain interaction between the N- and C-domains, thereby inhibiting the actin bundling activity of ABP34. This finding provides clues about the pathogenesis of neurodegenerative disorders related to actin bundling.

Effective non-denaturing purification method for improving the solubility of recombinant actin-binding proteins produced by bacterial expression

Bacterial expression is commonly used to produce recombinant and truncated mutant eukaryotic proteins. However, heterologous protein expression may render synthesized proteins insoluble. The conventional method used to express a poorly soluble protein, which involves denaturation and refolding, is time-consuming and inefficient. There are several non-denaturing approaches that can increase the solubility of recombinant proteins that include using different bacterial cell strains, altering the time of induction, lowering the incubation temperature, and employing different detergents for purification. In this study, we compared several non-denaturing protocols to express and purify two insoluble 34 kDa actin-bundling protein mutants. The solubility of the mutant proteins was not affected by any of the approaches except for treatment with the detergent sarkosyl. These results indicate that sarkosyl can effectively improve the solubility of insoluble proteins during bacterial expression.