The state of the filament

Movement is a defining characteristic of life. Macroscopic motion is driven by the dynamic interactions of myosin with actin filaments in muscle. Directed polymerization of actin behind the advancing membrane of a eukaryotic cell generates microscopic movement. Despite the fundamental importance of actin in these processes, the structure of the actin filament remains unknown. The Holmes model of the actin filament was published 15 years ago, and although it has been widely accepted, no high-resolution structural data have yet confirmed its veracity. Here, we review the implications of recently determined structures of F-actin-binding proteins for the structure of the actin filament and suggest a series of in silico tests for actin-filament models. We also review the significance of these structures for the arp2/3-mediated branched filament

The expanding superfamily of gelsolin homology domain proteins

The gelsolin homology (GH) domain has been found to date exclusively in actin-binding proteins. In humans, three copies of the domain are present in CapG, five copies in supervillin, and six copies each in adseverin, gelsolin, flightless I and the villins: villin, advillin and villin-like protein. Caenorhabditis elegans contains a four-GH-domain protein, GSNL-1. These architectures are predicted to have arisen from gene triplication followed by gene duplication to result in the six-domain protein. The subsequent loss of one, two or three domains produced the five-, four-, and three-domain proteins, respectively. Here we conducted BLAST and hidden Markov based searches of UniProt and NCBI databases to identify novel gelsolin domain containing proteins. The variety in architectures suggests that the GH domain has been tested in many molecular constructions during evolution. Of particular note is flightless-like I protein (FLIIL1) from Entamoeba histolytica, which combines a leucine rich repeats (LRR) domain, seven GH domains, and a headpiece domain, thus combining many of the features of flightless I with those of villin or supervillin. As such, the GH domain superfamily appears to have developed along complex routes. The distribution of these proteins was analyzed in the 343 completely sequenced genomes, mapped onto the tree of life, and phylogenetic trees of the proteins were constructed to gain insight into their evolution.ASTAR (Agency for Sci., Tech. and Research, S’pore

ATP competes with PIP₂ for binding to gelsolin

<div><p>Gelsolin is a severing and capping protein that targets filamentous actin and regulates filament lengths near plasma membranes, contributing to cell movement and plasma membrane morphology. Gelsolin binds to the plasma membrane via phosphatidylinositol 4,5-bisphosphate (PIP₂) in a state that cannot cap F-actin, and gelsolin-capped actin filaments are uncapped by PIP₂ leading to filament elongation. The process by which gelsolin is removed from PIP₂ at the plasma membrane is currently unknown. Gelsolin also binds ATP with unknown function. Here we characterize the role of ATP on PIP₂-gelsolin complex dynamics. Fluorophore-labeled PIP₂ and ATP were used to study their interactions with gelsolin using steady-state fluorescence anisotropy, and Alexa488-labeled gelsolin was utilized to reconstitute the regulation of gelsolin binding to PIP₂-containing phospholipid vesicles by ATP. Under physiological salt conditions ATP competes with PIP₂ for binding to gelsolin, while calcium causes the release of ATP from gelsolin. These data suggest a cycle for gelsolin activity. Firstly, calcium activates ATP-bound gelsolin allowing it to sever and cap F-actin. Secondly, PIP₂-binding removes the gelsolin cap from F-actin at low calcium levels, leading to filament elongation. Finally, ATP competes with PIP₂ to release the calcium-free ATP-bound gelsolin, allowing it to undergo a further round of severing.</p></div

Helix Straightening as an Activation Mechanism in the Gelsolin Superfamily of Actin Regulatory Proteins*

Villin and gelsolin consist of six homologous domains of the gelsolin/cofilin fold (V1–V6 and G1–G6, respectively). Villin differs from gelsolin in possessing at its C terminus an unrelated seventh domain, the villin headpiece. Here, we present the crystal structure of villin domain V6 in an environment in which intact villin would be inactive, in the absence of bound Ca2+ or phosphorylation. The structure of V6 more closely resembles that of the activated form of G6, which contains one bound Ca2+, rather than that of the calcium ion-free form of G6 within intact inactive gelsolin. Strikingly apparent is that the long helix in V6 is straight, as found in the activated form of G6, as opposed to the kinked version in inactive gelsolin. Molecular dynamics calculations suggest that the preferable conformation for this helix in the isolated G6 domain is also straight in the absence of Ca2+ and other gelsolin domains. However, the G6 helix bends in intact calcium ion-free gelsolin to allow interaction with G2 and G4. We suggest that a similar situation exists in villin. Within the intact protein, a bent V6 helix, when triggered by Ca2+, straightens and helps push apart adjacent domains to expose actin-binding sites within the protein. The sixth domain in this superfamily of proteins serves as a keystone that locks together a compact ensemble of domains in an inactive state. Perturbing the keystone initiates reorganization of the structure to reveal previously buried actin-binding sites

Model of the severing, capping, uncapping and inactivation/release cycle of gelsolin.

<p>The cartoon presents a model for the cycle of activation and function of gelsolin. Severing and capping: Elevated free calcium levels activate gelsolin, releasing ATP, leading to severing and capping of actin filaments. Depolymerization: Gelsolin-capped filaments will depolymerize from their pointed ends. Uncapping and polymerization: Gelsolin-capped actin filaments will be uncapped on encountering PIP₂ in the membrane, resulting in force being exerted on the membrane from the polymerization of the uncapped filaments. Inactivation and membrane release: Gelsolin will be released from PIP₂ and the membrane by competition with ATP. Following its release gelsolin is able to undergo subsequent cycles of severing, capping, uncapping and inactivation/release.</p

The interplay between calcium, magnesium, ATP and PIP₂ for binding to gelsolin.

<p><b>(a)</b> In the absence of cations, the anisotropy of ATP-N (0.5 μM) increased and saturated by ~ 5 μM gelsolin. Dashed line shows the fit to the data (<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0201826#pone.0201826.e002" target="_blank">Eq 2</a>). <b>(b)</b> Micromolar calcium levels (C, open triangles, inflection point at pCa 5.2) or millimolar magnesium levels (M, orange circles, inflection point at pMg 2.3) were able to reduce the anisotropy of ATP-N (0.5 μM) in the presence of gelsolin (5 μM), indicating the dissociation of ATP-N from gelsolin. Dashed line indicates sigmoidal fitting. <b>(c)</b> In the absence of cations, the anisotropy of PIP₂-F (0.5 μM) increased and reached steady-state at ~ 5 μM gelsolin, indicating saturation of binding. Dashed line shows the fit to the data (<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0201826#pone.0201826.e002" target="_blank">Eq 2</a>). <b>(d)</b> The anisotropy of PIP₂-F (0.5 μM) in the presence of gelsolin (5 μM) was not changed on titration with magnesium (M, orange circles), but inclusion of 0.5 mM ATP (MA, white circles) lowered the anisotropy to the value characteristic to free PIP₂-F, indicating complete dissociation of the gelsolin/PIP₂-F complex by ATP. This effect was diminished by magnesium concentrations above 7 mM (pMg 2.15). In the absence of ATP, PIP₂-F (0.5 μM) binds to gelsolin (5 μM), as revealed by the increase in anisotropy across a wide range of calcium concentrations (C, white triangles). Inclusion of ATP (0.2 mM, CA, purple triangles) lowered the anisotropy to the value characteristic to free PIP₂-F, indicating complete dissociation of the gelsolin/PIP₂-F complex by ATP.</p

ATP dependence of gelsolin binding to the surface of PIP₂-containing membrane vesicles.

<p><b>(a)</b> Gelsolin-Alexa488 (5 μM, red) was incubated with rhodamine590-filled, PIP₂-containing membrane vesicles (blue) in the absence of divalent cations and visualized by confocal microscopy. The merged image indicates that gelsolin and vesicles colocalized (top panel). After ATP (0.5 mM) was added, the majority of gelsolin-Alexa488 was released from the vesicles (middle panel). Following removal of ATP via buffer exchange and addition of fresh gelsolin-Alexa488 (15 μM), gelsolin re-associated with the vesicles (bottom panel). Scale bar = 10 μm. <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0201826#pone.0201826.s007" target="_blank">S1 Movie</a> details the time course of these changes. <b>(b)</b> Representative image of gelsolin-Alexa488 (15 μM, red) localized to the surface of a large rhodamine590-filled PIP₂-containing vesicle (blue) (top panel). After the addition of ATP (0.5 mM) the vesicle changed morphology concurrently with the release of gelsolin (red) from the vesicle surface (blue) (bottom panel). Scale bar = 10 μm. Panel <i>a</i> and <i>b</i> in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0201826#pone.0201826.s006" target="_blank">S6 Fig</a> show line scans across the images as indicated by the arrows and <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0201826#pone.0201826.s008" target="_blank">S2 Movie</a> details the time course. The confocal slice thickness was ~ 3 μm. There are two vesicles in focus, a small (d = 7 μm) and a large (d = 22.5 μm) one, the outer layer of the bigger vesicle is attached to the coverslip.</p

An ER-directed gelsolin nanobody targets the first step in amyloid formation in a gelsolin amyloidosis mouse model

Hereditary gelsolin amyloidosis is an autosomal dominantly inherited amyloid disorder. A point mutation in the GSN gene (G654A being the most common one) results in disturbed calcium binding by the second gelsolin domain (G2). As a result, the folding of G2 is hampered, rendering the mutant plasma gelsolin susceptible to a proteolytic cascade. Consecutive cleavage by furin and MT1-MMP-like proteases generates 8 and 5 kDa amyloidogenic peptides that cause neurological, ophthalmological and dermatological findings. To this day, no specific treatment is available to counter the pathogenesis. Using GSN nanobody 11 as a molecular chaperone, we aimed to protect mutant plasma gelsolin from furin proteolysis in the trans-Golgi network. We report a transgenic, GSN nanobody 11 secreting mouse that was used for crossbreeding with gelsolin amyloidosis mice. Insertion of the therapeutic nanobody gene into the gelsolin amyloidosis mouse genome resulted in improved muscle contractility. X-ray crystal structure determination of the gelsolin G2:Nb11 complex revealed that Nb11 does not directly block the furin cleavage site. We conclude that nanobodies can be used to shield substrates from aberrant proteolysis and this approach might establish a novel therapeutic strategy in amyloid diseases