Arabidopsis GNARLED Encodes a NAP125 Homolog that Positively Regulates ARP2/3

In migrating cells, the actin filament nucleation activity of ARP2/3 is an essential component of dynamic cell shape change and motility. In response to signals from the small GTPase Rac1, alterations in the composition and/or subcellular localization of the WAVE complex lead to ARP2/3 activation. The human WAVE complex subunit, WAVE1/SCAR1, was first identified in Dictyostelium and is a direct ARP2/3 activator. In the absence of an intact WAVE complex, SCAR/WAVE protein is destabilized. Although the composition of the five-subunit WAVE complex is well characterized, the means by which individual subunits and fully assembled WAVE complexes regulate ARP2/3 in vivo are unclear. The molecular genetics of trichome distortion in Arabidopsis is a powerful system to understand how signaling pathways and ARP2/3 control multicellular development. In this paper we prove that the GNARLED gene encodes a homolog of the WAVE subunit NAP125. Despite the moderate level of amino acid identity between Arabidopsis and human NAP125, both homologs were functionally interchangeable in vivo and interacted physically with the putative Arabidopsis WAVE subunit ATSRA1. gnarled trichomes had nearly identical cell shape and actin cytoskeleton phenotypes when compared to ARP2/3 subunit mutants, suggesting that GRL positively regulates ARP2/3.