The diversity of plant U-box E3 ubiquitin ligases: from upstream activators to downstream target substrates

Abstract

Ubiquitin-mediated proteolysis is an integral part of diverse cellular functions, and of the three enzymes involved in linking ubiquitin to protein targets, the E3 ubiquitin ligases are of particular interest as they confer substrate specificity during this process. The E3 ubiquitin ligases can be categorized based on mechanism of action and on the presence of specific domains such as RING, HECT, F-box, and U-box. In plants, the U-box family has undergone a large gene expansion that may be attributable to biological processes unique to the plant life cycle. For example, there are 64 predicted plant U-box (PUB) proteins in Arabidopsis, and the biological roles of many of these have yet to be determined. Research on PUB genes from several different plants has started to elucidate a range of functions for this family, from self-incompatibility and hormone responses to defence and abiotic stress responses. Expression profiling has also been used as a starting point to elucidate PUB function, and has uncovered a strong connection of PUB genes to various stress responses. Finally, some PUB proteins have been linked to receptor kinases as upstream activators, and downstream target substrates are also starting to emerge. The mechanisms of action range from the observation of mono-ubiquitination during non-proteolytic signalling to directed regulation of proteasomal components during stress responses, and cell death appears to be a theme underlying many PUB functions.

Changes in PUB22 Ubiquitination Modes Triggered by MITOGEN-ACTIVATED PROTEIN KINASE3 Dampen the Immune Response

“…However, since PUB22 and related genes are induced by infection and PAMP perception (Trujillo et al, 2008;Yee and Goring, 2009), transcriptional regulation may also contribute to elevated protein levels. Using transgenic Arabidopsis lines expressing a functional GFP-PUB22 fusion protein under the control of the native promoter (Supplemental Figure 2A), flg22 elicitation resulted in detectable protein levels only after 60 min, with no apparent basal protein expression (Supplemental Figure 2B).…”

A survey of genes involved in Arachis stenosperma resistance to Meloidogyne arenaria race 1

“…Likewise, plant genes containing the U-BOX (PUB) domain have been demonstrated to be upregulated under pathogen infection (Yee and Goring 2009) or after elicitor treatment, such as chitin, found in nematode exoskeleton (Libault et al 2007). The silencing of ubiquitin-related genes reduced the HR in tobacco and pepper, whereas its over-expression enhanced disease resistance in Arabidopsis (González-Lamothe et al 2006;Lee et al 2011).…”

Arabidopsis ubiquitin ligase MIEL1 mediates degradation of the transcription factor MYB30 weakening plant defence

“…HECT proteins are unique because they form an Ub-E3 intermediate before transfer of Ub to the substrate 7 . RING and U-Box proteins are structurally related single polypeptides that respectively use zinc chelation and hydrogen bonds/salt bridges to transfer Ub from the Ub-E2 intermediate to the target 8,9 . E3 ligases containing a RING domain can act independently or as part of a multisubunit cullin-RING ligase complex such as SCF (Skp1, Cullin, F-box-type ligase).…”