Ahg12 is a dominant proteasome mutant that affects multiple regulatory systems for germination of Arabidopsis.

Shimpei Hayashi,Takashi Hirayama

doi:10.1038/srep25351

Abstract

The ubiquitin-proteasome system is fundamentally involved in myriad biological phenomena of eukaryotes. In plants, this regulated protein degradation system has a pivotal role in the cellular response mechanisms for both internal and external stimuli, such as plant hormones and environmental stresses. Information about substrate selection by the ubiquitination machinery has accumulated, but there is very little information about selectivity for substrates at the proteasome. Here, we report characterization of a novel abscisic acid (ABA)-hypersensitive mutant named ABA hypersensitive germination12 (ahg12) in Arabidopsis. The ahg12 mutant showed a unique pleiotropic phenotype, including hypersensitivity to ABA and ethylene, and hyposensitivity to light. Map-based cloning identified the ahg12 mutation to cause an amino acid conversion in the L23 loop of RPT5a, which is predicted to form the pore structure of the 19S RP complex of the proteasome. Transient expression assays demonstrated that some plant-specific signaling components accumulated at higher levels in the ahg12 mutant. These results suggest that the ahg12 mutation led to changes in the substrate preference of the 26S proteasome. The discovery of the ahg12 mutation thus will contribute to elucidate the characteristics of the regulated protein degradation system.

Highlights

Between TIR1 and AUX/IAA and facilitates the degradation of this inhibitory protein to activate auxin-responsive gene expression
RPN10 and RPN13 have the ability to bind ubiquitin while RPN11 removes ubiquitins from the ubiquitinated proteins[21,22,23], suggesting that the lid subcomplex is involved in the recognition of ubiquitinated proteins
We found that the ahg[12] mutation alters an amino acid residue in RP AAA-ATPase 5a (RPT5a)

Summary

Discussion

We isolated ahg[12] as an ABA-related mutant that showed a unique combination of phenotypes, with increased ABA and ethylene sensitivity and decreased light sensitivity at germination and decreased seed dormancy. Our study on ahg[12] emphasizes the importance of protein degradation in the regulation of plant biological phenomena including germination, and confirms the necessity of the OB fold loop structure of the RP complex for proteasome activity. The transgenic plants overexpressing RPT5b with an ahg12-like mutation did not show ABA hypersensitivity (Fig. S6), suggesting that there are some functional differences between RPT5a and RPT5b. We plan to generate modified plants in which other RPT members have ahg12-like mutations and to evaluate the resulting phenotypes and proteasome substrate specificities. The identification of this dominant RPT5a mutant allele represents an important step toward addressing long-standing questions in proteasome biology

Methods

Findings

Additional Information