## 2 Structure of the USP Family
In bacteria, the functions of these classes are multifaceted but may overlap: Class I UspA and D protect against oxidative agents, while UspD is involved in the availability of intracellular iron; Class I UspC and Class II UspF and UspG play their roles in motility and adhesion, while Class III, Class IV and UspE convey overlaying functions with the other classes (Figure 1) [15]. USPs of the FG type contain the ATP binding site that is characterized by the conserved G-2X-G-9X-G-(S/T) motif [6; 34]. The binding of ATP serves as a functional regulator of USPs [35]. Leading to this discovery was the study of two crystal structures representative of each sub-family: UspA of _Haemophilus influenzae_ and the MJ0577 UspFG from _Methanococcus januschi_. Similarities have been observed in both their tertiary \(\alpha/\beta\) folds within the asymmetric dimer where adjacent \(\beta\)5 strands result in an anti-parallel interaction forming the interface of both proteins [26; 36]. The structural differences notable between the two proteins include their ATP binding activity. A study by [PERSON] and [PERSON] confirmed that UspA does not bind ATP despite the use of 1 mM MgATP included in crystal growth experiments. Tight ATP binding has however been observed in MJ0557 experiments despite the exclusion of ATP from purification and crystallization experiments. The stimulation of ATP hydrolysis by MJ0577 is suggestive of the protein functioning as a cell factor-dependent ATPase, which sheds more light on the biochemical activities of bacterial USPs. These still require more identification [37; 38]. Coordinating the binding of the ATP nucleotide are conserved residues: Gly127 that bind to ribose, Gly130 that binds to \(\beta\)-phosphate and Ser141 that binds to \(\gamma\)-phosphate by hydrogen bonding, respectively, while Gly14 interacts with the \(\alpha\)-phosphate and Asp13 and Val14 interacts with the base of ATP [14]. Apart from environmental stressors, the USP up-regulatory response is mediatedby the phosphorylation of serine and threonine residues by ATP and guanosine triphosphate, which are both phosphate donors [39]. Instead of glycine amino acids and the ATP binding motif, the UspA of \(H\). _influenza_, on the other hand, contains heavy side-chained residues such as glutamine and methionine. Since it lacks the nucleotide-binding motif, these proteins are likely to form a group of UspA-like ancestors. It is suggested that the two classes evolved so as to perform diverse functions within the cell [14]. Although the exact mechanism of action of USPs is largely evasive, their ubiquitous nature in various organisms, as well as their augmentation during conditions of stress, is a clear indication of their direct or indirect importance in providing resilience against these. Apart from plants, USPs are also present in disease-causing pathogens, therefore studying the structures and functions of these proteins could provide extra opportunities for understanding their involvement in parasite survival and persistent to eventually develop solutions to various diseases. The following sections shine a spotlight on current investigations pertaining to this.