BRICK1 requires SCAR for localization to the plasma membrane and is up-regulated by auxin

The SCAR/WAVE complex is an activator of the ARP2/3 complex, which initiates actin filament formation important for normal morphology in numerous cell types. BRK1 is a subunit of the SCAR/WAVE complex, and BRK1 fused to YFP in stable A. thaliana transformants localizes to the cell periphery, particularly at three-cell junctions. Plasmolysis experiments demonstrate that BRK1::YFP localization is associated with the plasma membrane, not with the cell wall. Despite this localization pattern, no reduction or disorganization of cortical actin filaments was observed via visualization of the actin-binding protein ABD2 fused to GFP in brk1 mutants. Treatment of cells with Brefeldin A, an inhibitor of the protein secretion pathway, also had no affect on BRK1::YFP localization, indicating that BRK1 is targeted through another mechanism. Instead, analysis of BRK1::YFP localization in scar1,2,3,4 mutants reveal that SCAR protein is required for BRK1 localization. Whether PtdIns(3,4,5)P₃, which is necessary for WAVE2 localization in animals, is also required for BRK1::YFP localization in A. thaliana could not be conclusively determined here. GUS reporter assays of SCAR2, SCAR3, and BRK1 show partially overlapping promoter activities, but that only the BRK1 promoter is activated by NAA, a member of the auxin hormone family. However, brk1 mutants showed no defects in the localization of the auxin transporter PIN1 fused to GFP. In addition, neither brk1 nor arp2 mutants had defects in phototropic bending, a process that involves auxi