Control of actin dynamics by proteins made of β-thymosin repeats. The Actobindin family

Abstract

Actobindin is an actin-binding protein from amoeba, which consists of two β-thymosin repeats and has been shown to inhibit actin polymerization by sequestering G-actin and by stabilizing actin dimers. Here we show that actobindin has the same biochemical properties as the Drosophila or Caenorhabditis elegans homologous protein that consists of three β-thymosin repeats. These proteins define a new family of actin-binding proteins. They bind G-actin in a 1:1 complex with thermodynamic and kinetic parameters similar to β-thymosins. Like β-thymosins, they slow down nucleotide exchange on G-actin and make a ternary complex with G-actin and Latrunculin A. On the other hand, they behave as functional homologs of profilin because their complex with MgATP-G-actin, unlike β-thymosin-actin, participates in filament barbed end growth, like profilin-actin complex. Therefore these proteins play an active role in actin-based motility processes. In addition, proteins of the actobindin family interact with the pointed end of actin filaments and inhibit pointed end growth, maybe via the interaction of the β-thymosin repeats with two terminal subunits.