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Link to original content: https://pubmed.ncbi.nlm.nih.gov/9334343
Evidence for a conformational change in actin induced by fimbrin (N375) binding - PubMed Skip to main page content
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. 1997 Oct 20;139(2):387-96.
doi: 10.1083/jcb.139.2.387.

Evidence for a conformational change in actin induced by fimbrin (N375) binding

D Hanein  1 , P Matsudaira, D J DeRosier
Affiliations

Evidence for a conformational change in actin induced by fimbrin (N375) binding

D Hanein et al. J Cell Biol. .

Abstract

Fimbrin belongs to a superfamily of actin cross-linking proteins that share a conserved 27-kD actin-binding domain. This domain contains a tandem duplication of a sequence that is homologous to calponin. Calponin homology (CH) domains not only cross-link actin filaments into bundles and networks, but they also bind intermediate filaments and some signal transduction proteins to the actin cytoskeleton. This fundamental role of CH domains as a widely used actin-binding domain underlines the necessity to understand their structural interaction with actin. Using electron cryomicroscopy, we have determined the three-dimensional structure of F-actin and F-actin decorated with the NH2-terminal CH domains of fimbrin (N375). In a difference map between actin filaments and N375-decorated actin, one end of N375 is bound to a concave surface formed between actin subdomains 1 and 2 on two neighboring actin monomers. In addition, a fit of the atomic model for the actin filament to the maps reveals the actin residues that line, the binding surface. The binding of N375 changes actin, which we interpret as a movement of subdomain 1 away from the bound N375. This change in actin structure may affect its affinity for other actin-binding proteins and may be part of the regulation of the cytoskeleton itself. Difference maps between actin and actin decorated with other proteins provides a way to look for novel structural changes in actin.

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Figures

Figure 2
Figure 2
(a) Surface presentation of the average F-actin data set. (b) Plot of Gn.l(R) amplitudes (solid lines) and phases (dotted lines) for the F-actin data set trimmed out to 28 Å. (c) Surface presentation of the average Actin-N375 data set. (d) Plot of Gn.l(R) amplitudes (solid lines) and phases (dotted lines) trimmed out to 28 Å (c). For plotting purposes only, amplitudes of weak layer lines were scaled by a factor of 2 and are marked with 2×. In all reconstructions, the pointed end of the actin filament is up. Bar, 50 Å.
Figure 1
Figure 1
(a) An electron micrograph of an ice-embedded actin filament that was included in the average data set. (b) Computed electron diffraction pattern of the filament shown in a. (c) An electron micrograph of an ice-embedded actin filament decorated with N375 that was included in the average data set. (d) Computed electron diffraction pattern of the filament shown in c. The location of the strong layer lines are indicated by arrows. Bar, 100 Å.
Figure 3
Figure 3
Sections through the F-actin reconstruction (F-actin) and the Actin-N375 reconstruction (Actin-N375). Sections are taken from the barbed end towards the pointed end. The sections are spaced 4.3 Å apart. The difference map between the two reconstructions is presented in the right column. The positive differences are contoured using black lines. Peak 1 corresponds to the N375 portion of the map, and peak 2 corresponds to the conformational change in subdomain 1. Contours corresponding to the actin map are superimposed on the difference contours. Contours start at the same density (4.0) in all maps. Contours are plotted at intervals of 6.3 in the F-actin and Actin-N375 maps and at intervals of 1.1 in the difference map. Bar, 50 Å.
Figure 4
Figure 4
(a) Electron density map of F-actin overlaid with the atomic model of F-actin (Lorenz et al., 1993). (b) The Actin-N375 electron density map (grid presentation) displayed in alignment with the F-actin map. (c) Ribbon presentation of two actin monomers in which the proposed N375-binding site is highlighted. (d) Stereo presentation of the positive difference peaks arising from subtracting the F-actin map from the Actin-N375 map are shown together with the atomic model of F-actin. The outer density peak corresponds to N375, and the inner peak corresponds to a conformational change observed in actin upon N375 binding. In all maps, the pointed end of the actin filament is up.
Figure 5
Figure 5
Schematic diagram of one actin subunit in the F-actin filament (a). The conformational change induced by N375 binding to F-actin is schematically presented in b. Subdomain 1 is shifted outward and downward in a direction away from the bound N375.
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