A Per-ARNT-Sim-Like Sensor Domain Uniquely Regulates the Activity of the Homeodomain Leucine Zipper Transcription Factor REVOLUTA inArabidopsis

Enrico Magnani,M Kathryn Barton

doi:10.1105/tpc.110.080754

Abstract

Class III homeodomain leucine zipper (HD-ZIP III) transcription factors regulate critical developmental programs in plants; these include leaf polarity, polarity along the shoot-root axis, and stem cell specification and proliferation. One of the defining features of HD-ZIP III proteins is the presence of a Per-ARNT-Sim-like (PAS-like) MEKHLA domain at the C terminus. PAS-like domains are known to respond to a variety of chemical and physical stimuli. Here, we provide evidence that the MEKHLA domain acts as a negative regulator of Arabidopsis thaliana HD-ZIP III REVOLUTA activity. Based on experiments in yeast and plants, we propose a model in which the MEKHLA domain inhibits dimerization through a sequence-independent steric masking mechanism. This inhibition is relieved in response to a cellular signal that requires the C terminus of the MEKHLA domain for its perception. Overexpression experiments suggest that this signal is unequally distributed and/or sensed in the plant. Our data show that the function of the REVOLUTA MEKHLA domain differs among other HD-ZIP III family members; this difference may explain the genetic differences that have been observed among family members. This finding, combined with our phylogenetic analysis, suggests that REVOLUTA is the latest type of HD-ZIP III protein to have evolved in land plants.

Highlights

Class III homeodomain leucine zipper proteins (HD-ZIP III proteins) are plant-specific transcription factors that play prominent roles in plant development
The MEKHLA domain is a conserved domain located at the C terminus of all HD-ZIP III proteins (Mukherjee and Burglin, 2006)
The N-terminal flanking region has weak similarity to the WD-40 domain; we named it MEKHLA WD40-like (M-WD40)

Summary

Introduction

Class III homeodomain leucine zipper proteins (HD-ZIP III proteins) are plant-specific transcription factors that play prominent roles in plant development. Dimerization of HD-ZIP III proteins in vivo is regulated by a family of small proteins that consist almost entirely of leucine zipper sequence (Wenkel et al, 2007; Kim et al, 2008). These LITTLE ZIPPER (ZPR) proteins interact with HD-ZIP III proteins in vitro and prevent DNA binding. Overexpression of ZPR proteins causes phenotypes similar to those seen in loss-of-function HDZIP III mutants. These observations are the basis for a model in which ZPR proteins act to negatively regulate HD-ZIP III activity by preventing their dimerization

Methods

Results

Conclusion