Abstract
Ubiquitination is a major type of post-translational modification of proteins in eukaryotes. The plant U-Box (PUB) E3 ligase is the smallest family in the E3 ligase superfamily, but plays a variety of essential roles in plant growth, development and response to diverse environmental stresses. Hence, PUBs are potential gene resources for developing climate-resilient crops. However, there is a lack of review of the latest advances to fully understand the powerful gene family. To bridge the gap and facilitate its use in future crop breeding, we comprehensively summarize the recent progress of the PUB family, including gene evolution, classification, biological functions, and multifarious regulatory mechanisms in plants.
Highlights
The National Key Facility for Crop Gene Resources and Genetic Improvement, Institute of Crop Sciences, International Maize and Wheat Improvement Center, Texcoco 56237, Mexico
A total of 118 plant U-Box (PUB) genes show drought- and heat-stress-specific expression patterns in wheat [15], 60 genes are induced by drought/salt/low temperature in banana [20], 11 out of 67 HvPUB genes are at least two-fold induced by drought in barley seedlings [13], supporting the extensive involvements of PUBs in abiotic stress responses
40% of irrigated lands worldwide are affected by increased salt levels, and the expansion of soil salinization is a serious threat to crop performance [56]
Summary
The U-box is a more recently identified protein domain with E3 ligase activity, which was first found in ubiquitin fusion degradation protein 2 (UFD2) in yeast [5,6]. The. U-box domain is a modified RING-finger domain, lacking the zinc-chelating cysteine and histidine residues of RING-finger domains [7]. The conserved zinc-binding sites (cysteine and histidine residues) supporting the cross-brace arrangement in RING-finger domains are replaced by hydrogen-bonding networks in the U-box. The U-box scaffold is stabilized by a system of salt bridges and hydrogen bonds. The charged and polar residues participating in the network of bonds are more conserved than those in classic RING fingers, revealing their irreplaceable role in maintaining the stability of U-box [7]. Val→Ile substitution disrupts structural integrity of the U-box and leads to pre-mRNA splicing defects of Prp19p [8]. Both U-box and RING fingers contain a conserved interaction surface. In Arabidopsis, identical amino acids are present in the corresponding conservative positions in the U-box domains of AtPUB14 and many other plant U-box proteins [9]
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