Building bridges: formin1 of Arabidopsis forms a connection between the cell wall and the actin cytoskeleton

Abstract

SUMMARYActin microfilament (MF) organization and remodelling is critical to cell function. The formin family of actin binding proteins are involved in nucleating MFs in Arabidopsis thaliana. They all contain formin homology domains in the intracellular, C-terminal half of the protein that interacts with MFs. Formins in class I are usually targeted to the plasma membrane and this is true of Formin1 (AtFH1) of A. thaliana. In this study, we have investigated the extracellular domain of AtFH1 and we demonstrate that AtFH1 forms a bridge from the actin cytoskeleton, across the plasma membrane and is anchored within the cell wall. AtFH1 has a large, extracellular domain that is maintained by purifying selection and that contains four conserved regions, one of which is responsible for immobilising the protein. Protein anchoring within the cell wall is reduced in constructs that express truncations of the extracellular domain and in experiments in protoplasts without primary cell walls. The 18 amino acid proline-rich extracellular domain that is responsible for AtFH1 anchoring has homology with cell-wall extensins. We also have shown that anchoring of AtFH1 in the cell wall promotes actin bundling within the cell and that overexpression of AtFH1 has an inhibitory effect on organelle actindependant dynamics. Thus, the AtFH1 bridge provides stable anchor points for the actin cytoskeleton and is probably a crucial component of the signalling response and actin-remodelling mechanisms.

Formins at a glance

“…Inverted formin-2 (INF2) and formin-like protein 2 (FMNL2, also known as FRL3) are also farnesylated and myristoylated, respectively, which promotes their membrane targeting (Chhabra et al, 2009;Block et al, 2012). Moreover, mDia1 and mDia2, as well as the plant formins AFH1, formin1 and class II formin, directly bind phospholipid membranes (Cheung et al, 2010;Ramalingam et al, 2010;Gorelik et al, 2011;Martinière et al, 2011;van Gisbergen et al, 2012). These observations show that localization and activation of formins depend on their diverse interactions and that localization and activation, in some cases, serve as convergent inputs from multiple signalling pathways.…”

Heterodimeric Capping Protein from Arabidopsis Is a Membrane-Associated, Actin-Binding Protein

“…This prediction was verified for Arabidopsis Formin1 (AtFH1), AtFH6, and AtFH5 (Banno and Chua, 2000;Cheung and Wu, 2004;Favery et al, 2004;Ingouff et al, 2005). The subcellular localization of AtFH1 was shown by fluorescent protein fusion to be targeted to the plasma membrane and colocalized with a known plasma membrane protein, aquaporin, plasma membrane intrinsic protein PIP2;1 (Martinière et al, 2011). AtFH6 has been shown to be uniformly distributed throughout the plasma membrane and to generate actin cables that serve as tracks for vesicle trafficking for extensive plasma membrane and cell wall biogenesis (Favery et al, 2004).…”

The cell wall of Arabidopsis thaliana influences actin network dynamics

“…Firstly, if the actin bundles observed in plasmolysed cells are involved in supporting the protoplast structure, then they would need to be relatively rigid and static. Secondly, Martinière et al (2011) suggested that the cell wall itself acts as a provider of stable anchor points for the dynamic actin cytoskeleton to remodel around, they used formin1 (AtFH1) as an example of a protein that binds both actin microfilaments and the cell wall and is involved in remodelling of the actin cytoskeleton (Figure 4). …”

“…Volocity software [PerkinElmer, Massachusetts, USA] was used to track Golgi body movement as has been described previously (Runions et al, 2006;Sparkes et al, 2008;Martinière et al, 2011;Rocchetti et al, 2014).…”

“…For example, Martinière et al (2011) over-expressed AtFH1 and observed a very dense meshwork of actin cytoskeleton composed of finer filaments with fewer bundles and they determined that Golgi bodies had reduced motility. In addition, Akkerman et al (2011) imaged Golgi bodies moving on actin and found that they moved at greater speeds and in a more directional manner on actin bundles compared to on fine filaments.…”