Abstract
Transformations between G- (monomeric) and F-actin (polymeric) are important in cellular behaviors such as migration, cytokinesis, and morphing. In order to understand these transitions, we combined single-molecule Förster resonance energy transfer with total internal reflection fluorescence microscopy to examine conformational changes of individual actin protomers. We found that the protomers can take different conformational states and that the transition interval is in the range of hundreds of seconds. The distribution of these states was dependent on the environment, suggesting that actin undergoes spontaneous structural changes that accommodate itself to polymerization.
Highlights
Actin, one of the most abundant proteins in eukaryotes, plays a key role in many cellular processes such as migration and morphing by switching between its monomeric (G-actin) and polymeric (F-actin) forms [1]
G-F transformation can be regulated by a variety of actin-binding proteins (ABPs) at physiological conditions [12], recent data detailing the structures suggest that G-F transformation involves large conformational changes in actin protomers independent of ABP [13]
On the basis of the FRET efficiencies, we surmised that the g and fg states in G-actin are analogous to the g and fg states in Factin, while the f state is unique to F-actin (Fig. 3)
Summary
One of the most abundant proteins in eukaryotes, plays a key role in many cellular processes such as migration and morphing by switching between its monomeric (G-actin) and polymeric (F-actin) forms [1]. While the crystallographic structure of G-actin has been solved in various conditions [3,4,5,6], the equivalents for F-actin are not known. Recent cryo-electron microscopy (cryo-EM) studies have suggested structural models for F-actin; one of them showed a single well-defined structure [9,10], and another suggested structural polymorphisms [11]. Actin polymerization proceeds when the concentration of Gactin exceeds a critical concentration, which is defined as the concentration of monomers that achieves a dynamic equilibrium for actin filament polymerization and depolymerization. G-F transformation can be regulated by a variety of actin-binding proteins (ABPs) at physiological conditions [12], recent data detailing the structures suggest that G-F transformation involves large conformational changes in actin protomers independent of ABP [13]. The existence of an intermediate state of G-actin preceding the transformation in solution, called F-monomer [14], G*-actin [15], and KClmonomer [16], have been suggested
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