Abstract
The actin cytoskeleton has the particularity of being assembled into many functionally distinct filamentous networks from a common reservoir of monomeric actin. Each of these networks has its own geometrical, dynamical and mechanical properties, because they are capable of recruiting specific families of actin-binding proteins (ABPs), while excluding the others. This review discusses our current understanding of the underlying molecular mechanisms that cells have developed over the course of evolution to segregate ABPs to appropriate actin networks. Segregation of ABPs requires the ability to distinguish actin networks as different substrates for ABPs, which is regulated in three different ways: (1) by the geometrical organization of actin filaments within networks, which promotes or inhibits the accumulation of ABPs; (2) by the identity of the networks' filaments, which results from the decoration of actin filaments with additional proteins such as tropomyosin, from the use of different actin isoforms or from covalent modifications of actin; (3) by the existence of collaborative or competitive binding to actin filaments between two or multiple ABPs. This review highlights that all these effects need to be taken into account to understand the proper localization of ABPs in cells, and discusses what remains to be understood in this field of research.
Highlights
Actin plays a major role in many different biological processes such as cytokinesis, migration, vesicular trafficking and infection [1,2]
It is important to note that all these proteins coexist in the cell cytoplasm, only a specific subset of actinbinding proteins (ABPs) interacts with each actin network while being excluded from the others [5,6,7]
While ADF/cofilin is clearly localized on branched actin networks such as endocytic actin patches, ADF/cofilin is hardly detectable on linear networks such as actin cables [37,38,39]. These observations indicate that additional principles, beyond the actin filament network architecture, need to be taken into account to obtain a global picture of how ABPs are addressed in cells
Summary
Actin plays a major role in many different biological processes such as cytokinesis, migration, vesicular trafficking and infection [1,2]. Triggering actin assembly from cellular extracts by specific factors such as WASp (which is an activator of the Arp2/3 complex) or formins, leads to the formation of actin filament networks with a composition of ABPs comparable to branched and linear actin networks, respectively [12,13,14] All of these observations unambiguously indicate that, to a large extent, actin networks themselves represent different substrates for downstream protein interactions. It is important to note that the sensitivity of ABPs to different actin filament organizations does not seem to be limited to the case of molecular motors, but seems, on the contrary, to be quite general Crosslinkers such as α-actinin bind preferentially when the spacing between two actin filaments is favourable [35]. These observations indicate that additional principles, beyond the actin filament network architecture, need to be taken into account to obtain a global picture of how ABPs are addressed in cells
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