Arabidopsis BRICK1/HSPC300 is an essential WAVE-complex subunit that selectively stabilizes the Arp2/3 activator SCAR2

The actin cytoskeleton dynamically reorganizes the cytoplasm during cell morphogenesis. The actin-related protein (Arp)2/3 complex is a potent nucleator of actin filaments that controls a variety of endomembrane functions including the endocytic internalization of plasma membrane [1], vacuole biogenesis [2, 3], plasma-membrane protrusion in crawling cells [4], and membrane trafficking from the Golgi [5]. Therefore, Arp2/3 is an important signaling target during morphogenesis. The evolutionarily conserved Rac-WAVE-Arp2/3 pathway links actin filament nucleation to cell morphogenesis [6-9]. WAVE translates Rac-GTP signals into Arp2/3 activation by regulating the stability and/or localization of the activator subunit Scar/WAVE [8, 10-12]. The WAVE complex includes Sra1/PIR121/ CYFIP1, Nap1/NAP125, Abi-1/Abi-2, Brick1(Brk1)/ HSPC300, and Scar/WAVE [10, 13]: Defining the in vivo function of each subunit is an important step toward understanding this complicated signaling pathway. Brk1/HSPC300 has been the most recalcitrant WAVE-complex protein and has no known function. In this paper, we report that Arabidopsis brick1 (brk1) is a member of the distorted group of trichome morphology mutants, a group that defines a WAVE-ARP2/3 morphogenesis pathway [14]. In this paper we provide the first strong genetic and biochemical evidence that BRK1 is a critical WAVE-complex subunit that selectively stabilizes the Arp2/3 activator SCAR2.