Many different actin-binding proteins are responsible for the creation and turnover of actin filaments, as well as their assembly into higher-order structures, such as actin bundles, which have a number of roles in the cell. Although much information has been gleaned about the functions of actin bundles, little is known about how they form. Recent work has described the LIM proteins, which constitute a new family of small actin-bundling proteins that are widely expressed in plants. Now (p. 583), Clément Thomas and colleagues investigate the mechanisms of actin bundling by the tobacco LIM-domain-containing protein WLIM1 (NtWLIM1) in both in vitro reconstituted assays and live cells. The authors first show that NtWLIM1 can bind to individual actin filaments and promotes actin bundling. The NtWLIM1-induced bundles of mixed polarity elongate using two distinct mechanisms: growing at their terminal regions by the addition of pre-formed short filaments and polymerization of the inside filaments. The authors next use complementary live-cell-imaging approaches to establish a close relationship between NtWLIM1 subcellular localisation and self-association along the actin cytoskeleton, and show that LIM domains are involved in NtWLIM1 self-association. Moreover, when deprived of an intact actin cytoskeleton, NtWLIM1 exhibits a reduced ability to form complexes, suggesting that an intact actin cytoskeleton is required for the formation and/or stabilisation of the NtWLIM1 complex. The authors therefore propose a multistep model in which F-actin-induced self-association of NtWLIM1 drives the zippering of NtWLIM1-decorated actin filaments.

• © 2014. Published by The Company of Biologists Ltd