The plant formin AtFH4 interacts with both actin and microtubules, and contains a newly identified microtubule-binding domain

The dynamic behaviour of the actin cytoskeleton in plants relies on the coordinated action of several classes of actin-binding proteins (ABPs). These ABPs include the plant-specific subfamilies of actin-nucleating formin proteins. The model plant species Arabidopsis thaliana has over 20 formin proteins, all of which contain plant-specific regions in place of the GTPase-binding domain, formin homology (FH)3 domain, and DAD and DID motifs found in many fungal and animal formins. We have identified for the first time a plant-specific region of the membrane-integrated formin AtFH4 that mediates an association with the microtubule cytoskeleton. In vitro analysis shows that this region (named the GOE domain) binds directly to microtubules. Overexpressed AtFH4 accumulates at the endoplasmic reticulum membrane and co-aligns the endoplasmic reticulum with microtubules. The FH1 and FH2 domains of formins are conserved in plants, and we show that these domains of AtFH4 nucleate F-actin. Together, these data suggest that the combination of plant-specific and conserved domains enables AtFH4 to function as an interface between membranes and both major cytoskeletal networks.