The structure of gelsolin has been determined by crystallography and comprises six structurally related domains that, in a Ca2+-free environment, pack together to form a compact globular structure in which the putative actin-binding sequences are not sufficiently exposed to enable binding to occur. We propose that binding Ca2+ can release the connections that join the N- and C-terminal halves of gelsolin, enabling each half to bind actin relatively independently. Domain shifts are proposed in response to Ca2+ as bases for models of how gelsolin acts to sever, cap, or nucleate F-actin filaments. The structure also invites discussion of polyphosphoinositide binding to segment 2 and suggests how mutation at Asp-187 could initiate a series of events that lead to deposition of amyloid plaques, as observed in victims of familial amyloidosis (Finnish type).

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The WH2 (Wiscott-Aldridge syndrome protein homology domain 2) repeat is an actin interacting motif found in monomer sequestering and filament assembly proteins. We have stabilized the prototypical WH2 family member, thymosin-b4 (Tb4), with respect to actin, by creating a hybrid between gelsolin domain 1 and the C-terminal half of Tb4 (G1-Tb4). This hybrid protein sequesters actin monomers, severs actin filaments and acts as a leaky barbed end cap. Here, we present the structure of the G1-Tb4:actin complex at 2 Å resolution. The structure reveals that Tb4 sequesters by capping both ends of the actin monomer, and that exchange of actin between Tb4 and profilin is mediated by a minor overlap in binding sites. The structure implies that multiple WH2 motifcontaining proteins will associate longitudinally with actin filaments. Finally, we discuss the role of the WH2 motif in arp2/3 activation.

Gelsolin superfamily members are Ca 21 -dependent, multidomain regulators of the actin cytoskeleton. Calcium binding activates gelsolin by inducing molecular gymnastics (large-scale conformational changes) that expose actin interaction surfaces by releasing a series of latches. A specialized tail latch has distinguished gelsolin within the superfamily. Active gelsolin exhibits actin filament severing and capping, and actin monomer sequestering activities. Here, we analyze a combination of sequence, structural, biophysical and biochemical data to assess whether the molecular plasticity, regulation and actinrelated properties of gelsolin are also present in other superfamily members. We conclude that all members of the superfamily will be able to transition between a compact conformation and a more open form, and that most of these open forms will interact with actin. Supervillin, which lacks the severing domain 1 and the F-actin binding-site on domain 2, is the clear exception. Eight calcium-binding sites are absolutely conserved in gelsolin, adseverin, advillin and villin, and compromised to increasing degrees in CapG, villin-like protein, supervillin and flightless I. Advillin, villin and supervillin each contain a potential tail latch, which is absent from CapG, adseverin and flightless I, and ambiguous in villin-like protein. Thus, calcium regulation will vary across the superfamily. Potential novel isoforms of the superfamily suggest complex regulation at the gene, transcript and protein levels. We review animal, clinical and cellular data that illuminate how the regulation of molecular flexibility in gelsolin-like proteins permits cells to exploit the force generated from actin polymerization to drive processes such as cell movement in health and disease. V C 2013 Wiley Periodicals, Inc.

The actin filament-severing functionality of gelsolin resides in its N-terminal three domains (G1-G3). We have determined the structure of this fragment in complex with an actin monomer. The structure reveals the dramatic domain rearrangements that activate G1-G3, which include the replacement of interdomain interactions observed in the inactive, calcium-free protein by new contacts to actin, and by a novel G2-G3 interface. Together, these conformational changes are critical for actin filament severing, and we suggest that their absence leads to the disease Finnish-type familial amyloidosis. Furthermore, we propose that association with actin drives the calcium-independent activation of isolated G1-G3 during apoptosis, and that a similar mechanism operates to activate native gelsolin at micromolar levels of calcium. This is the first structure of a filament-binding protein bound to actin and it sets stringent, high-resolution limitations on the arrangement of actin protomers within the filament.

The actin-binding protein gelsolin is involved in remodeling the actin cytoskeleton during growth-factor signaling, apoptosis, cytokinesis, and cell movement. Calcium-activated gelsolin severs and caps actin filaments. The 3.4 angstrom x-ray structure of the carboxyl-terminal half of gelsolin (G4-G6) in complex with actin reveals the basis for gelsolin activation. Calcium binding induces a conformational rearrangement in which domain G6 is flipped over and translated by about 40 angstroms relative to G4 and G5. The structural reorganization tears apart the continuous beta sheet core of G4 and G6. This exposes the actin-binding site on G4, enabling severing and capping of actin filaments to proceed.

Gelsolin consists of six homologous domains (G1-G6), each containing a conserved Ca-binding site. Occupation of a subset of these sites enables gelsolin to sever and cap actin filaments in a Ca-dependent manner. Here, we present the structures of Ca-free human gelsolin and of Ca-bound human G1-G3 in a complex with actin. These structures closely resemble those determined previously for equine gelsolin. However, the G2 Ca-binding site is occupied in the human G1-G3/actin structure, whereas it is vacant in the equine version. In-depth comparison of the Ca-free and Ca-activated, actin-bound human gelsolin structures suggests G2 and G6 to be cooperative in binding Ca 2؉ and responsible for opening the G2-G6 latch to expose the F-actin-binding site on G2. Mutational analysis of the G2 and G6 Ca-binding sites demonstrates their interdependence in maintaining the compact structure in the absence of calcium. Examination of Ca binding by G2 in human G1-G3/actin reveals that the Ca 2؉ locks the G2-G3 interface. Thermal denaturation studies of G2-G3 indicate that Ca binding stabilizes this fragment, driving it into the active conformation. The G2 Ca-binding site is mutated in gelsolin from familial amyloidosis (Finnish-type) patients. This disease initially proceeds through protease cleavage of G2, ultimately to produce a fragment that forms amyloid fibrils. The data presented here support a mechanism whereby the loss of Ca binding by G2 prolongs the lifetime of partially activated, intermediate conformations in which the protease cleavage site is exposed.actin ͉ calcium activated ͉ calcium dependent ͉ TIRF

Cellulomonas fimi endo-beta-1,4-glucanase A (CenA) contains a discrete N-terminal cellulose-binding domain (CBDCenA). Related CBDs occur in at least 16 bacterial glycanases and are characterized by four highly conserved Trp residues, two of which correspond to W14 and W68 of CBDCenA. The adsorption of CBDCenA to crystalline cellulose was compared with that of two Trp mutants (W14A and W68A). The affinities of the mutant CBDs for cellulose were reduced by approximately 50- and 30-fold, respectively, relative to the wild type. Physical measurements indicated that the mutant CBDs fold normally. Fluorescence data indicated that W14 and W68 were exposed on the CBD, consistent with their participation in binding to cellobiosyl residues on the cellulose surface.