Update on the Ubiquitin Modification System Ubiquitin on the Move: The Ubiquitin Modification System Plays Diverse Roles in the Regulation of Endoplasmic Reticulum- and Plasma Membrane-Localized Proteins

Ubiquitin ligation to other proteins modulates the activity, longevity, and/or localization of the target proteins in eukaryotic systems. As components of the ubiquitin pathway, plant hormone receptors determine the abundance of key transcriptional regulators of auxin, GA, and jasmonate response pathways (for review, see Kelley and Estelle, 2012). Ubiquitin has also been shown to alter the abundance, function, and localization of membraneand endoplasmic reticulum (ER)-resident proteins. Other hormone signaling cascades, including the ethylene and brassinosteroid pathways, depend on the regulated proteolysis of membrane-associated receptor-like protein kinases. Significantly, the consequences of ubiquitin modification in some cases are independent of the proteasome. An appropriate general term, inclusive of all processes involving ubiquitin, is the ubiquitin modification system (UMS). This Update, in addition to reviewing general aspects of the UMS, specifically focuses on the roles of ubiquitin in the plant endomembrane system.