The Expanding Universe of Ubiquitin and Ubiquitin-Like Modifiers

The post-translation addition of ubiquitin (Ub) and structurally related polypeptide modifiers to other intracellular constituents is now known to be an important intermediate driving a wide assortment of metabolic and regulatory processes in plants. Although the most common targets are other proteins, conjugation of specific modifiers to lipids and prenyl groups or participation in sulfur chemistry, tRNA modification, and RNA splicing also occur. The Ub system is clearly the most complex in terms of targets (~1000s) and underpinning enzymatic machinery (~6% of the Arabidopsis proteome), with its primary role to initiate the selective turnover of unwanted proteins by the 26S proteasome. However, recent studies revealed that several other modifiers also direct critical plant functions, especially with respect to autophagic recycling, the regulation of transcription and chromatin structure, and RNA metabolism. In fact, emerging studies have unearthed an intricate interplay between Ub and several of the Ub-like modifiers that sometimes works in synergy with or against ubiquitylation. While first thought to be restricted to eukaryotes, proteins related in structure and/or biochemistry to the Ub superfamily have been found in eubacteria and Archaea, indicating that this modification scheme evolved very early in cellular life, with prokaryotic sulfur transferase chemistry being the likely antecedent. Given their widespread distribution, diversity of targets, and multifaceted activities, Ub-type proteins are fast assuming their rightful places as fundamental regulators of plant biology.