The Ubiquitin Conjugating Enzyme: An Important Ubiquitin Transfer Platform in Ubiquitin-Proteasome System

Abstract

Owing to a sessile lifestyle in nature, plants are routinely faced with diverse hostile environments such as various abiotic and biotic stresses, which lead to accumulation of free radicals in cells, cell damage, protein denaturation, etc., causing adverse effects to cells. During the evolution process, plants formed defense systems composed of numerous complex gene regulatory networks and signal transduction pathways to regulate and maintain the cell homeostasis. Among them, ubiquitin-proteasome pathway (UPP) is the most versatile cellular signal system as well as a powerful mechanism for regulating many aspects of the cell physiology because it removes most of the abnormal and short-lived peptides and proteins. In this system, the ubiquitin-conjugating enzyme (E2) plays a critical role in transporting ubiquitin from the ubiquitin-activating enzyme (E1) to the ubiquitin-ligase enzyme (E3) and substrate. Nevertheless, the comprehensive study regarding the role of E2 enzymes in plants remains unexplored. In this review, the ubiquitination process and the regulatory role that E2 enzymes play in plants are primarily discussed, with the focus particularly put on E2's regulation of biological functions of the cell.

Keywords: E2 functions; ubiquitin; ubiquitin-conjugating enzyme (E2); ubiquitin-proteasome system.

Figure 1

The ubiquitin-proteasome system. The process of ubiquitination from activation to attachment to the substrate is catalyzed by three major enzymes. The substrates labeled by ubiquitin are degraded by the 26S proteasome or play a non-degradative role in other processes. Abbreviations: APC, Anaphase-promoting complex; DUBs, Deubiquitinating enzymes; E1, Ubiquitin-activating enzyme; E2, Ubiquitin-conjugating enzyme; E3, Ubiquitin-ligase enzyme; Cul-based, Cullin-RING box1-Ligase; HECT, Homology to E6-AP C Terminus; Ub, Ubiqitin; SUMO, Small ubiquitin-related modifier; RBX1, RING-Box 1; RING, Really interesting new gene; RBR, RING1-IBR(cysteine/histidine rich region)-RING2.

Figure 2

The role of UBC13 in DNA damage responses and UBC13′s other functions. (A) UBC13 is involved in post-replication repair (PRR). (B) UBC13 participates in different biological processes by mediating Lys63-linked ubiquitination. Abbreviations: Aux/IAA, Auxin/indole-3-acetic acid; DDT, DNA damage tolerance; ERAD, ER-associated degradation; Mms2, A Ubc E2 variant; PRR, Postreplication repair; PCNA, Proliferating cell nuclear antigen; RGLG1/2, An E3 ligase; RNF5, RING finger protein 5; Rad5, An E3 enzyme; Rad6, An E2 enzyme; Rad18, An E3 enzyme; Suv, Fni3 (Fen-interacting protein, Fen is the host protein kinase) cofactor S. lycoperiscum Uev; Siz1, An E3 ligase; Srs2, A helicase; TLS, Translesion DNA synthesis; Ubc9, An E2 enzyme.