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. 2001 May 28;153(5):947–956. doi: 10.1083/jcb.153.5.947

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© 2001 The Rockefeller University Press

This article is distributed under the terms of an Attribution–Noncommercial–Share Alike–No Mirror Sites license for the first six months after the publication date (see http://www.rupress.org/terms). After six months it is available under a Creative Commons License (Attribution–Noncommercial–Share Alike 4.0 Unported license, as described at http://creativecommons.org/licenses/by-nc-sa/4.0/).

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Possible assembly mechanism of actin–fimbrin arrays. Whether a straight or a slanted crossband appearance occurs depends on which ABD binds first to F-actin (top and bottom). ABD1 of fimbrin is pink; ABD2, blue; and the calcium-binding domain, cyan (with color scheme as described in the legend to Fig. 3). The affinity of the actin-binding sites is color coded in the following manner: gray denotes low affinity; yellow, somewhat higher affinity; and orange, the highest affinity. Note that all arguments are equally valid if ABD1 and ABD2 are exchanged within the model (e.g., ABD1 would be blue, ABD2 pink) or if the straight crossband conformation (top) is flipped 180° around the paper plane. (A) Two F-actin fragments and one unbound fimbrin molecule. (B) The fimbrin molecule is now bound to one of the F-actin fragments (with low affinity; gray), triggering higher affinity at the opposite binding site (yellow). This is presumably achieved by conformational changes in actin. A second unbound fimbrin is also shown. (C) The previously unbound fimbrin molecule is now bound to the binding site with higher affinity, triggering now higher affinity at the original first binding site (now also in yellow). (D) The second previously unbound F-actin fragment is now bound to the second fimbrin molecule with low affinity. This can only happen in a relatively fixed geometry due to the limited flexibility of the fimbrin molecule. The binding again triggers higher affinity at the opposite binding site of this filament (yellow). A third unbound fimbrin molecule is now shown. (E) The binding of the last unbound fimbrin to the binding site triggers higher affinity at the opposite previously occupied binding site on the same filament. The fimbrin molecule that is bound to both actin filaments may also trigger even higher affinities at the two contacted binding sites (orange).