Ubiquitination of the ribosomal-like domain of UBIQUITIN6 contributes to cell wall stress responses in Arabidopsis

Plants must constantly sense the status of their cell walls and respond by modifying cell walls to maintain growth and respond to developmental and external stimuli. This capacity to sense cell walls and mount appropriate responses are collectively called cell wall signaling. Reversible post-translational modifications, such as phosphorylation or ubiquitination, are often an important part of signal transduction since they can rapidly and reversibly modify protein function and/or localization. To uncover differential post-translational modifications during cell wall stress, we analyzed the ubiquitinated proteome of Arabidopsis seedlings treated with the cell wall synthesis inhibitor, isoxaben. We identified 49 proteins that are significantly differentially ubiquitinated under short-term isoxaben treatment. We examined the phenotypes of loss-of-function mutants affecting one of these candidates, UBQ6, and found that ubq6 mutants are relatively resistant to isoxaben. This resistance is partially conveyed by sustained cellulose synthesis during isoxaben treatment. We further found that UBQ6 is a ubiquitin extension protein family member and that it is cleaved into two domains: the ubiquitin-like domain and the ribosomal-like domain. We confirmed that the site that is differentially ubiquitinated under cell wall stress is within the ribosomal-like domain of UBQ6 and found that the ribosomal-like domain is required for wild-type cell wall stress responses. We propose a model in which ubiquitination of the ribosomal-like domain of a UBQ6 has a specific role in cell wall signaling and cell wall stress responses, independent of the ubiquitin molecule created by UBQ6 protein cleavage.