Figure 6. Auxin stimulates PTRE1 accumulation at the plasma membrane.

(a) Western blot analysis showed that auxin treatment results in relatively more PTRE1 at the plasma membrane, but decreased levels in nucleus and cytoplasm. Arabidopsis seedlings expressing PTRE1-GFP were treated with NAA (1 μM, 60 or 120 min) and then surface-exposed membrane proteins were analysed using PTRE1 antibody or plasma membrane marker H⁺-ATPase. Nucleus and cytoplasm fractions were prepared from Col seedlings and analysed by western blot using antibody against PTRE1, nuclear marker H3 or cytoplasm marker UGPase. The relative quantities of the proteins were calculated by using image pro plus and indicated. (b) A proposed model for how PTRE1 and TIR1 coordinate auxin responses regulating proteasome activity and Aux/IAA protein degradation. Under normal condition (with basal auxin levels), 26S proteasome activity is maintained by appropriate distribution of PTRE1 at the plasma membrane and in intracellular compartments. In response to auxin, auxin rapidly stimulates the association of TIR1 and Aux/IAA proteins resulting in degradation of Aux/IAAs (1). Later, auxin suppresses PTRE1 to inhibit proteasome activity (possibly through stimulating the accumulation of PTRE1 at plasma membrane, resulting in decreased intracellular and nuclear localization) and hence suppresses Aux/IAA protein degradation (2) to coordinate regulation of auxin responses, reflecting a mechanism for fine control of Aux/IAA homeostasis and auxin signalling.