Modulation of Phototropic Responsiveness in Arabidopsis through Ubiquitination of Phototropin 1 by the CUL3-Ring E3 Ubiquitin Ligase CRL3NPH3  

Abstract

Plant phototropism is an adaptive response to changes in light direction, quantity, and quality that results in optimization of photosynthetic light harvesting, as well as water and nutrient acquisition. Though several components of the phototropic signal response pathway have been identified in recent years, including the blue light (BL) receptors phototropin1 (phot1) and phot2, much remains unknown. Here, we show that the phot1-interacting protein NONPHOTOTROPIC HYPOCOTYL3 (NPH3) functions as a substrate adapter in a CULLIN3-based E3 ubiquitin ligase, CRL3 NPH3 . Under low-intensity BL, CRL3 NPH3 mediates the mono/multiubiquitination of phot1, likely marking it for clathrin-dependent internalization from the plasma membrane. In high-intensity BL, phot1 is both mono/multi-and polyubiquitinated by CRL3 NPH3 , with the latter event targeting phot1 for 26S proteasome-mediated degradation. Polyubiquitination and subsequent degradation of phot1 under high-intensity BL likely represent means of receptor desensitization, while mono/multiubiquitination-stimulated internalization of phot1 may be coupled to BL-induced relocalization of hormone (auxin) transporters.

The Signal Transducer NPH3 Integrates the Phototropin1 Photosensor with PIN2-Based Polar Auxin Transport in Arabidopsis Root Phototropism

“…Our results show that all lines have similar gravitropic bending Shaded areas are based on the early hypothesis , while bright areas are based on our results to explain PIN2 localization and dynamics in darkness and under BL illumination. Membrane-localized phot1 acts as a BL photoreceptor to activate the signal transducer scaffold protein NPH3 (Wan et al, 2008;Roberts et al, 2011). NPH3 plays role in switching the dynamic equilibrium of PIN2 between PVC-targeting trafficking and membrane-targeting recycling.…”

Plasma membrane protein ubiquitylation and degradation as determinants of positional growth in plants

“…Direct evidence for ubiquitylation acting as principal signal for PM protein endocytosis was provided by analysis of cargo-ubiquitin fusion proteins (Herberth et al 2012;Leitner et al 2012a;Scheuring et al 2012). In some instances, environmental cues appear to modulate PM protein ubiquitylation, thereby affecting protein sorting in response to specific signals (Kasai et al 2011;Roberts et al 2011;Leitner et al 2012aLeitner et al , 2012b. Nevertheless, vacuolar targeting for degradation might as well occur independently of PM cargo ubiquitylation (Cai et al 2012).…”

“…Nevertheless, it is tempting to speculate about mechanisms, in which variations in ubiquitylation patterns would control discrete aspects of PIN2 intracellular traffic, as already suggested for blue light receptor protein (Roberts et al 2011; see below, Figure 1). Characterization of trans-acting regulators as well as a detailed analysis of PIN2 ubiquitylation modes will be required, to further test this hypothesis.…”

“…In both concepts, expression and activity of phot1 would have a decisive role in the regulation of PAT, and recent work demonstrated phot1 control by its ubiquitylation. Blue light-controlled variations in phot1 ubiquitylation have been linked to its intracellular localization and turnover, with phot1 mono-ubiquitylation triggering its clathrin-dependent endocytosis, whereas highintensity blue light causes phot1 poly-ubiquitylation that was suggested to signal proteasome-mediated degradation of the photoreceptor (Roberts et al 2011). It is not fully understood, how light-dependent variations in phot1 ubiquitylation could contribute to its suggested activities in the regulation of auxin transporters.…”

“…Thus, a unifying picture of the respective roles in the different steps of downregulation of membrane proteins is still lacking (Sigismund et al 2004;d'Azzo et al 2005;Lauwers et al 2009). In addition, deubiquitylating enzymes (DUBs) also contribute to the modulation of the ubiquitin signals either removing, or trimming it, thereby reversing or reinforcing the pathway ( (Roberts et al 2011), and IRT1 iron carrier (Barberon et al 2011). Direct evidence for ubiquitylation acting as principal signal for PM protein endocytosis was provided by analysis of cargo-ubiquitin fusion proteins (Herberth et al 2012;Leitner et al 2012a;Scheuring et al 2012).…”