Formins Determine the Functional Properties of Actin Filaments in Yeast

Abstract

The actin cytoskeleton executes a broad range of essential functions within a living cell. The dynamic nature of the actin polymer is modulated to facilitate specific cellular processes at discrete locations by actin-binding proteins (ABPs), including the formins and tropomyosins (Tms). Formins nucleate actin polymers, while Tms are conserved dimeric proteins that form polymers along the length of actin filaments. Cells possess different Tm isoforms, each capable of differentially regulating the dynamic and functional properties of the actin polymer. However, the mechanism by which a particular Tm localizes to a specific actin polymer is unknown. Here we show that specific formin family members dictate which Tm isoform will associate with a particular actin filament to modulate its dynamic and functional properties at specific cellular locations. Exchanging the localization of the fission yeast formins For3 and Cdc12 results in an exchange in localizations of Tm forms on actin polymers. This nucleator-driven switch in filament composition is reflected in a switch in actin dynamics, together with a corresponding change in the filament's ability to regulate ABPs and myosin motor activity. These data establish a role for formins in dictating which specific Tm variant will associate with a growing actin filament and therefore specify the functional capacity of the actin filaments that they create.

Tropomyosin – master regulator of actin filament function in the cytoskeleton

“…1) (Johnson et al, 2014). This model fits well with previous findings (Martin et al, 2010) because it explains how Tpm sorting can occur in the context of actin-Tpm copolymerization on a growing actin filament.…”

“…Furthermore, there has been direct in vivo evidence from the fission yeast model system (Johnson et al, 2014). This yeast contains a single Tpm (Cdc8), which exists in either an N-terminally acetylated (80% of total Tpm) or non-acetylated state (Johnson et al, 2014), with the acetylated Tpm associating with actin filaments that are incorporated into the cytokinetic actomyosin ring during mitosis and the unacetylated form associating with more-dynamic actin filaments during interphase (Skoumpla et al, 2007;Coulton et al, 2010). Indeed, forcing the mitotic and interphase formins to switch location, led to a corresponding switch in the acetylated state of Tpms within each actin-Tpm polymer, which -in turn -was shown to redirect the location of the myosin motors in the cell (Johnson et al, 2014).…”

“…This model fits well with previous findings (Martin et al, 2010) because it explains how Tpm sorting can occur in the context of actin-Tpm copolymerization on a growing actin filament. Furthermore, there has been direct in vivo evidence from the fission yeast model system (Johnson et al, 2014). This yeast contains a single Tpm (Cdc8), which exists in either an N-terminally acetylated (80% of total Tpm) or non-acetylated state (Johnson et al, 2014), with the acetylated Tpm associating with actin filaments that are incorporated into the cytokinetic actomyosin ring during mitosis and the unacetylated form associating with more-dynamic actin filaments during interphase (Skoumpla et al, 2007;Coulton et al, 2010).…”

The evolution of compositionally and functionally distinct actin filaments

“…The principle of contractile force generated by the interaction of myosin II motors with actintropomyosin co-polymers is as ancient as the yeast contractile ring. The mechanism by which these compositionally distinct filaments, in terms of their tropomyosin content, are generated in fission yeast has recently been identified (Johnson et al, 2014). The two fission yeast formins generate actin filaments with different tropomyosin isoform compositions, and hence, with different functional properties.…”

“…The two fission yeast formins generate actin filaments with different tropomyosin isoform compositions, and hence, with different functional properties. Manipulation of a formin to a new location in the cell led to the assembly of actin filaments complete with the forminspecific tropomyosin at the new site (Johnson et al, 2014).…”