The yeast Rad p radiation sensitive also
The yeast Rad6p (radiation sensitive 6, also called Ubc2) E2 ubiquitin-conjugating enzyme is a member of the highly conserved UBC proteins in eukaryotes (Xu et al., 2009). Rad6p is involved in several cellular processes, including DNA repair, transcriptional activation and silencing, histone ubiquitination, ubiquitin-mediated N-end rule protein degradation and endoplasmic reticulum-associated protein degradation (ERAD) (Popovic et al., 2014, van Wijk and Timmers, 2010). Mutations in RAD6 homologs are involved in many diseases (Popovic et al., 2014). In Arabidopsis, 37 proteins with a UBC domain and active-site cysteine have been predicted. Among these Arabidopsis UBC proteins, the best-characterized RAD6 homolog is AtUBC2 (Xu et al., 2009). AtUBC2 can partially rescue the UV-sensitivity and slow growth of rad6Δ yeast (Zwirn et al., 1997). AtUbc2p has E2 activity in vitro and has been shown to mono-ubiquitinate substrates in the absence of E3 ubiquitin ligases (Strzalka et al., 2013). UBC2 is proposed to affect DNA repair, histone ubiquitination, flowering time, and enhance salt and drought-tolerance and modulate abiotic stress-induced gene Okadaic acid in plants (Cao et al., 2008, Qin, 2013, Xu et al., 2009). TBSV is a small (+)RNA virus that has been intensively used to study virus replication, recombination, and virus – host interactions based on yeast (Saccharomyces cerevisiae) model host (Nagy and Pogany, 2006, Nagy and Pogany, 2012, Nagy et al., 2014, Panavas and Nagy, 2003, Panaviene et al., 2004, White and Nagy, 2004). The auxiliary p33 replication protein, which is an RNA chaperone, recruits the TBSV (+)RNA to the cytosolic surface of peroxisomal membranes for replication (Jonczyk et al., 2007, McCartney et al., 2005, Nagy et al., 2012, Panavas et al., 2005a, Pogany et al., 2005, Stork et al., 2011). The interaction between the viral p33 and p92pol RdRp protein is required for assembling the functional VRC (Panavas et al., 2005a, Panaviene et al., 2004, Panaviene et al., 2005, Pogany and Nagy, 2008, Pogany and Nagy, 2012). The assembly and functions of VRCs are also affected by host components, such as the host heat shock protein 70 (Hsp70), the eukaryotic elongation factor 1A (eEF1A), ESCRT (endosomal sorting complexes required for transport) proteins and sterols and phospholipids (Li et al., 2008, Li and Nagy, 2011, Li et al., 2009, Li et al., 2010, Pogany and Nagy, 2012, Pogany and Nagy, 2015, Pogany et al., 2008, Serva and Nagy, 2006, Sharma et al., 2010, Sharma et al., 2011, Wang et al., 2009a, Wang et al., 2009b, Xu and Nagy, 2015). Genome-wide screens to identify host factors affecting TBSV RNA replication in yeast led to the identification of host genes known to be involved in various aspects of protein ubiquitination, such as BRE1, DOA4, RAD6, LGE1, UBP3 (Jiang et al., 2006, Panavas et al., 2005b, Serviene et al., 2005, Serviene et al., 2006). In addition, proteomics approach has revealed interaction between p33 replication protein and Uba1p ubiquitin- (Ub)-activating enzyme, Cdc34p E2 Ub-conjugating enzyme, Rsp5p E3 Ub-ligase, Ubp10p and Ubp15p Ub-specific proteases (Li et al., 2008). More detailed analysis with Cdc34p E2 Ub-conjugating enzyme showed that Cdc34p is present in the tombusvirus replicase complex and it can mono- and bi-ubiquitinate p33 in vitro in the absence of an E3 Ub-ligase (Li et al., 2008). The Nedd4-type Rsp5p E3 Ub-ligase has also been shown to bind to and ubiquitinylate p33 replication protein in vitro (Barajas et al., 2009b). Studies with the proteasomal Rpn11p metalloprotease, which acts as a deubiquitination (DUB) enzyme, has shown the role of Rpn11p in the assembly of TBSV VRCs, and the recruitment of the cellular DDX3-like Ded1p DEAD-box helicase into the viral replicase (Prasanth et al., 2014). Data also support the role of Rpn11p and the free ubiquitin pool in TBSV replication and viral RNA recombination (Prasanth et al., 2014). Altogether, the emerging idea from these studies on TBSV that ubiquitin and protein ubiquitination is a major element in virus replication and evolution.