Tropomyosin-binding properties modulate competition between tropomodulin isoforms

Abstract

The formation and fine-tuning of cytoskeleton in cells are governed by proteins that influence actin filament dynamics. Tropomodulin (Tmod) regulates the length of actin filaments by capping the pointed ends in a tropomyosin (TM)-dependent manner. Tmod1, Tmod2 and Tmod3 are associated with the cytoskeleton of non-muscle cells and their expression has distinct consequences on cell morphology. To understand the molecular basis of differences in the function and localization of Tmod isoforms in a cell, we compared the actin filament-binding abilities of Tmod1, Tmod2 and Tmod3 in the presence of Tpm3.1, a non-muscle TM isoform. Tmod3 displayed preferential binding to actin filaments when competing with other isoforms. Mutating the second or both TM-binding sites of Tmod3 destroyed its preferential binding. Our findings clarify how Tmod1, Tmod2 and Tmod3 compete for binding actin filaments. Different binding mechanisms and strengths of Tmod isoforms for Tpm3.1 contribute to their divergent functional capabilities.

Actin regulation by tropomodulin and tropomyosin in neuronal morphogenesis and function

“…Tmod2’s LRR domain has a structure similar to Tmod1, but was less resistant to denaturation by urea and more susceptible to protease degradation (Guillaud et al, 2014). The difference in stability of Tmod1’s and Tmod2’s LRR domains may be explained by difference in the orientation of the C-terminal α-helix, which is crucial for the LRR domain’s ability to bind actin (Colpan et al, 2016; Kostyukova and Hitchcock-DeGregori, 2004). These differences may contribute to the difference in affinity for actin monomers.…”

“…Disruption of Tpm’s coiled coil near the N-terminus by either mutagenesis or removing the acetyl group from the 1a exon of Tpms reduces actin binding (Moraczewska et al, 2000) and Tmod binding (Greenfield and Fowler, 2002). This is especially important for HMW Tpms; however, Tmods can effectively cap actin filaments decorated with LMW non-acetylated Tpms (Colpan et al, 2016; Kostyukova and Hitchcock-DeGregori, 2004; Kostyukova et al, 2007). Inhibition of the N-terminal acetyltransferase B complex (NatB), which acetylates Tpms, causes loss of actin stress fibers (Van Damme et al, 2012).…”

“…The interaction between Tmod and Tpm enhances the capping many fold (Weber et al, 1994). The affinities of Tmods’ binding sites vary between isoforms (Colpan et al, 2016; Uversky et al, 2011; Yamashiro et al, 2010). This gives rise to isoform specific variances in binding to different Tpm-decorated actin filaments (Colpan et al, 2016; Yamashiro et al, 2014).…”

“…The affinities of Tmods’ binding sites vary between isoforms (Colpan et al, 2016; Uversky et al, 2011; Yamashiro et al, 2010). This gives rise to isoform specific variances in binding to different Tpm-decorated actin filaments (Colpan et al, 2016; Yamashiro et al, 2014). For instance, Tmod3 binds to the pointed ends of actin filaments decorated with Tpm3.1 more tightly than either Tmod1 or Tmod2 (Colpan et al, 2016).…”

“…This gives rise to isoform specific variances in binding to different Tpm-decorated actin filaments (Colpan et al, 2016; Yamashiro et al, 2014). For instance, Tmod3 binds to the pointed ends of actin filaments decorated with Tpm3.1 more tightly than either Tmod1 or Tmod2 (Colpan et al, 2016). This variation in binding affinities give rise to additional degrees of complexity in actin regulation as Tmods can non-exclusively bind onto many types of filaments in a cellular subdomain.…”