Polypeptide tags, ubiquitous modifiers for plant protein regulation

Evidence has emerged over the past few years that plants, like animals and fungi, employ a variety of polypeptides as tags to reversibly or irreversibly affect the function, structure, location, and/or turnover of numerous intracellular proteins. In plants, known polypeptide tags include ubiquitin, SUMO, RUB, and APG12, with the possibility of others. These modifiers are typically added post-translationally using individual sets of conjugase pathways that attach the polypeptides via an isopeptide bond to ε-lysyl amino group(s) in the targets. Often the tags can be removed subsequently by unique proteases that specifically cleave only the isopeptide bond. Examples also exist where the tag is added during translation upon fusion of the coding sequence of the tag with that of the target. Based on the number and diversity of targets, ubiquitin is the most influential modifier which mainly serves as a reusable signal for selective protein degradation by the 26S proteasome. In contrast, SUMO, RUB and APG12 become attached to a more limited number of targets and appear to have specialized functions, including roles in nuclear pore assembly/function, cell-cycle regulation, and lysosomal/vacuole trafficking, respectively. Based on their widespread occurrence in plants and their pervasive role in various biological processes, polypeptide tags likely play a prominent role in plant cell regulation.