Auxin stimulates gene expression by triggering the proteasome-mediated proteolysis of a family of transcriptional repressors, auxin/indole acetic acid (Aux/IAA) proteins. Tao et al. showed that in transfected protoplasts expressing constitutively active or dominant-negative forms of the guanosine triphosphatase (GTPase) Rac, the activity of Rac controlled the degradation of luciferase-Aux/IAA fusion proteins (Luc-Aux/IAA). Constitutively active Rac stimulated degradation even in the absence of auxin. Furthermore, transfection of GTPase-activating proteins (GAPs) inhibited auxin-stimulated degradation of Luc-Aux/IAA. In protoplasts transfected with green fluorescent protein (GFP)-Aux/IAA fusion proteins under the control of an inducible promoter, the GFP signal was predominantly nuclear, and addition of auxin or coexpression of constitutively active Rac accelerated the rate at which the GFP signal became undetectable following cessation of expression. Dominant-negative Rac stabilized GFP-Aux/IAA. In protoplasts and in roots from seedling plants, auxin treatment triggered the redistribution of the GFP-Auz/IAA proteins from a diffuse nuclear signal to one associated with subnuclear puncta called nuclear protein bodies (NPBs). Inhibition of the proteasome with MG132 or expression of dominant-negative Rac prevented the auxin-induced redistribution of GFP-Aux/IAA to NPBs. Although auxin initially promoted the accumulation of GFP-Aux/IAA into NPBs, ultimately the GFP signal became undetectable, suggesting that these were degradative macromolecular structures. Consistent with this degradative role, auxin also stimulated the accumulation of constituents of the SCF E3 ligases, the COP9 signalosome, and the proteasome in the NPBs when these were coexpressed with a labeled Aux/IAA protein. These results together suggest that auxin acts through Rac to trigger the localization of Aux/IAA proteins into NPBs, which serve as nuclear sites of degradation.
L.-z. Tao, A. Y. Cheung, C. Nibau, H.-m. Wu, RAC GTPases in tobacco and arabidopsis mediate auxin-induced formation of proteolytically active nuclear protein bodies that contain AUX/IAA proteins. Plant Cell 17, 2369-2383 (2005). [Abstract] [Full Text]