Review
The many implications of actin filament helicity

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Abstract

One of the best known features of actin filaments is their helical structure. A number of essential properties emerge from this molecular arrangement of actin subunits. Here, we give an overview of the mechanical and biochemical implications of filament helicity, at different scales. In particular, a number of recent studies have highlighted the role of filament helicity in the adaptation to and the generation of mechanical torsion, and in the modulation of the filament’s interaction with very different actin-binding proteins (such as myosins, cross-linkers, formins, and cofilin). Helicity can thus be seen as a key factor for the regulation of actin assembly, and as a link between biochemical regulators and their mechanical context. In addition, actin filament helicity appears to play an essential role in the establishment of chirality at larger scales, up to the organismal scale. Altogether, helicity appears to be an essential feature contributing to the regulation of actin assembly dynamics, and to actin’s ability to organize cells at a larger scale.

Section snippets

Introduction: helical structure of the actin filament

Helicity is a common feature in biopolymers. In the case of actin filaments, helicity is an essential characteristic which can be linked to several key properties of the filaments. We begin here by introducing the main parameters and numbers used to describe actin filament helicity. The following sections will address the mechanical implications of actin filament helicity (Section 2), the role of helicity in the interactions of filaments with actin-binding proteins (Section 3) and the role it

Mechanical implications of actin filament helicity

Before discussing how ABPs can modulate filament helicity (Section 3) and what consequences it can have at larger scales (Section 4), let us briefly summarize some of the physical consequences of helicity.

An obvious consequence of helicity is that it gives filaments an inherent periodicity, other than that of the subunits themselves. This simple feature imposes geometrical constraints when packing filaments into tight bundles (Section 3.1).

Another natural consequence of helicity is that it

Crosstalk between helicity and ABPs

The fact that actin filaments are helical has consequences for several, very different ABPs. They are both affected by helicity and able to modify it. Helicity and the mechanical constraints modifying helicity are thus important parameters for the regulation of actin assembly and the organization of filament networks.

From actin filament helicity to cellular and organismal chiralities

Over the past years, filament helicity and helicity-sensitive ABPs have been linked to the generation of chirality and asymmetry at larger scales, which are fundamental aspects of life. The establishment of left-right asymmetry, in particular, is an important and challenging question. In development, left-right asymmetry appears after anterior/posterior and dorsal/ventral asymmetries, which are determined at the time of fertilization. Proteins and most biological molecules are chiral (and actin

Conclusion and future directions

As we have discussed in Section 3, actin filament helicity is sensed and modified by various types of ABPs. Helicity thus provides an essential handle for ABPs to modulate each other’s regulatory actions. Interestingly, ABPs’ ability to affect the filament’s helicity depends strongly on the filaments’ geometrical organization. For example, details of filament and formin anchoring have an impact on the generation of torque, and on the action of regulatory proteins such as cofilin. This provides

Declaration of Competing Interest

The authors declare no conflict of interest.

Acknowledgements

We acknowledge funding from the French ANR (Grants ‘MuscActin’ and ‘Conformin’) to G.R.-L. and the European Research Council (Grant StG-679116 to A.J.).

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