Viral enzymes are less specific to dUTP than the bacterial one
Viral enzymes are less specific to dUTP than the bacterial one [65]. The MMTV enzyme has a reduced discrimination against dTTP and UTP, while it is still selective against dCTP. Despite the relatively large body of research conducted on beta-retroviral dUTPases, we could not find any studies on the biological importance of the enzyme to the life cycle and infectivity of the viruses. However, this missing information can be complemented by the studies described below on involvement of dUTPases in the infectivity of the non-primate lentiviruses.Lentiviruses Nonprimate lentivirusesAmong all exogenous lentiviruses, dUTPase-encoding genes were observed in only the non-primate lentiviruses. However, there are traits of these genes in some ERVs, including in many HERVs (see below). Amongthe non-primate lentiviruses, dUTPase-encoding genes are present in feline immunodeficiency virus (FIV) [66], puma lentivirus [67], equine infectious anemia virus (EIAV) [68, 69], caprine arthritis-encephalitis virus (CAEV) [70] and visna virus of sheep [71]. Additionally, in the bovine lentiviruses, BIV that is associated with a debilitating cattle SIS3 cancer disease [72], and Jembrana disease virus (JDV), a homologous dUTPase-encoding gene is present [73, 74], see above (Fig. PubMed ID:https://www.ncbi.nlm.nih.gov/pubmed/28494239 5). In a rare case of the infectious small-ruminant genotype E lentivirus (isolated from goats and sheep), almost the entire dUTPase genome is deleted [75]. As mentioned above, the dUTPase encoding gene in this retroviral group is part of the pol gene and is situated between the RT and IN encoding segments (Fig. 3). Another feature that sets the non-primate lentiviral dUTPases apart from the beta-retroviral counterparts (and from other known dUTPases as well) is the relatively shorter polypeptide subunit, of about 130 PubMed ID:https://www.ncbi.nlm.nih.gov/pubmed/26740125 residues (which is roughly half of the beta-retroviral dUTPases). Despite this major difference, similar to other studied dUTPases, the three-dimensional structure of EIAV dUTPase exhibits a homo-trimeric arrangement, where each subunit folds into a twisted antiparallel betabarrel with the N and C-terminal portions interacting with the adjacent subunits [76]. A generally similar structure was reported also for FIV dUTPase [77], see also Fig. 4. The majority of the biochemical studies on the dUTPases of the non-primate lentiviruses were conducted on recombinant EIAV dUTPase. This enzyme was shown to be highly specific to dUTP and sensitive to inhibition by dUDP, with little inhibition by other nucleotides or the reaction products, dUMP and PPi [78]. In this study, mutational analyses were also performed by targeting a conserved domain present at the C-terminus of all dUTPases. This domain shares high homology with the phosphate binding loops (P-loops) of several ATP and GTP-binding phosphatases. The P-loop-like motif of dUTPases is glycine rich, but lacks the invariant lysine found in authentic P-loops. Deletion of this motif led to a loss of the enzymatic activity. In addition, a series of point mutations in EIAV dUTPase that inactivate these P-loops also abolished the dUTPase activity; thus establishing the importance of these loops for catalysis. Another study compared EIAV dUTPase with the E. coli counterpart [79]. The results showed that the viral enzyme was as potent as the bacterial one in hydrolyzing dUTP, albeit less specific. The inhibition of the EIAV enzyme by dTTP, dUMP and a synthetic analog is stronger by one order of magnitude than that of the bacterial counterpart. T.
