br Acknowledgments This work was supported
Acknowledgments This work was supported by a grant from the National Center for GM Crops (PJ008152) of the Next Generation BioGreen 21 Program funded by the Rural Development Administration, Republic of Korea, to W.T.K.
Introduction Ubiquitin (Ub) has been considered as one of the most important regulatory proteins in eukaryotes since it was first discovered in 1978 (Rechsteiner, 1987; Severo et al., 2013; Collins and Brown, 2010; Jiang and Chen, 2011; Wenzel et al., 2011). Ubiquitination is an enzymatic process including three sequential steps performed by the ubiquitin-activating enzyme (E1), the ubiquitin-conjugating enzymes (E2) and the ubiquitin-ligases (E3) (Dupre et al., 2004; Jiang and Chen, 2011; Wenzel et al., 2011), which involves in the bonding of the Ub to target proteins through the covalent attachment to regulate a multitude of cellular processes. The ubiquitination starts with the activation of Ub by E1 in an ATP-dependent reaction to generate an E1∼Ub conjugate, and then the activated Ub is handed over to an E2 via a transthiolation reaction. The E2∼Ub conjugate selectively interacts with E3s that recruits and binds to specific substrates (Hershko et al., 1982; Wijk and Timmers, 2010; Stewart et al., 2016). E2 is the key components of Ub transfer pathway in ubiquitination, which is responsible for E3 selection and substrate modification as well as the diversity of Ub cellular signaling (Huang et al., 2009; Sakata et al., 2010). E2s can be easily identified by their evolutionarily conserved ‘Ubc’ catalytic domain with 150–200 amino glasdegib clinical residues. Ubc domain has a compact structure shaped like a prolate ellipsoid, which is comprised of four α-helices, a short 310 helix near the active site, and a four-stranded antiparallel β-sheet (Whitcomb and Taylor, 2009; Sakata et al., 2010; Stewart et al., 2016). In addition to Ubc domain, most of E2s contain variable N- or C-terminal extensions appended to the catalytic domain. E2 family has been divided into four classes based on the existence of additional extensions, Class I containing the Ubc domain only, Class II possessing the Ubc domain and a C-terminal extension, Class III possessing the Ubc and an N-terminal extension, and Class IV consisting of Ubc domain plus both N and C-terminal extensions (Zhao et al., 2007; Huang et al., 2009; Whitcomb et al., 2009). Accumulating evidences have demonstrated that E2s play key roles in controlling the life process of the eukaryotic cell such as degradation of misfolded proteins, cell cycle progression, and DNA repair (Nascimento et al., 2006). Recently, E2s have also been found to involve in many other biological processes, such as diseases of the nervous system, cancer development, apoptosis as well as immune response (Kim et al., 2005; Ueki et al., 2009; Stewart et al., 2016). Ubiquitination has been reported to play a central role in innate immunity, in which E2s are critical for recognition of invading pathogens and activation of inflammation (Collins and Brown, 2010; Jiang and Chen, 2011; Severo et al., 2013). Most of these E2s involved in immune response belong to the Class I UBE2D (Ubc5), UBE2G, and UBE2N (Ubc13) family (Fukushima et al., 2007; Stewart et al., 2016). For example, UBE2N (Ubc13) could promote the Tax-ms mediated activation of NF-κB, and bind the RING domains of TNF receptor associated factor to mediate TNF receptor and Toll-like receptor signaling (Shembade et al., 2007; Choy et al., 2013). Increasing evidences have indicated that E2s are also closely related to the immune defenses of invertebrates. The Ubc5, Ubc13 and Uev1a from Drosophila were critical components of the immune deficiency (IMD) pathway and mediated the activation of TGF-β activated kinase (TAK1) and NF-κB inhibitor kinase (IKK) complex (Paquette et al., 2010). A novel UBE2D homolog ab-UBE2D was identified in Haliotis diversicolor supertexta and it was involved in the immune response (Ye et al., 2013). Interestingly, E2s in medically-important arthropods (Effete, Bendless and Uev1a) are also utilized by some pathogens to cause prevalent vector-borne diseases, including malaria, dengue fever, Lyme disease, and shigellosis (Kim et al., 2005; Choy et al., 2013; Severo et al., 2013), indicating that E2s are crucial factors for Ub dependent host-pathogen interactions. However, the role of E2s in the interactions between lower invertebrates and pathogen is still not known.