 a wide variety of cellular processes ([@bib32]). In most cases, the carboxy terminus of ubiquitin (Ub) is covalently linked to the ε-amino (a primary amine) group of one or more lysines on substrates. Subsequent additions of further Ub moieties create polymers of Ub, which have diverse structures and functions ([@bib73]). These Ub structures can be recognized by specific receptors that contain Ub-binding domains (UBDs) that can result in the delivery of ubiquitinated substrate to the proteasome for degradation or to selective autophagy pathways to changes in protein function and/or cellular localization ([@bib21]). The mechanism underlying the ubiquitination process is well established. It involves a cascade of three enzymes: E1-Ub activating enzyme, E2-Ub conjugating enzyme, and E3-Ub ligase. Specific enzymes called deubiquitinases (DUBs) cleave off Ub from substrates and regulate the abundance of ubiquitinated proteins ([@bib19]).

Given the importance of Ub signaling, a considerable number of pathogens utilize virulence factors that modulate Ub and autophagy systems to promote pathogenicity ([@bib29], [@bib31], [@bib42], [@bib44], [@bib52]). This is clearly demonstrated by *Legionella pneumophila*, a Gram-negative bacterium that causes Legionnaires' disease and possesses the largest number of documented bacterial effectors among intracellular bacterial pathogens ([@bib16]). For example, the *Legionella* effectors LegU1 and LeuAU13 serve as F-box-containing E3 ligases that interact with host Cul1-Skp1 and ubiquitinate BAT3, a host chaperone protein ([@bib27]). Another effector is LubX, a RING and U-box type E3 ligase, which, in conjunction with the host E2 enzymes UbcH5a or UbcH5c, ubiquitinates host Clk1 kinase ([@bib41], [@bib55]). More recently, *Legionella pneumophila* was also shown to utilize a non-canonical type of ubiquitination through the action of enzymes belonging to the SidE family of effectors (SdeA, SdeB, SdeC, and SidE) ([@bib13], [@bib53]). This NAD-dependent modification involves the conjuga