IDENTIFICATION OF A STRUCTURALLY AND EVOLUTIONARILY CONSERVED 3'→5' PROOFREADING EXONUCLEASE DOMAIN IN THE LARGE L PROTEIN OF HANTAVIRUSES- EMERGING HUMAN PATHOGENS

The proofreading 3′→5′ exonuclease not only represents a crucial class of enzymes responsible for maintaining genome integrity across diverse groups of organisms, but also plays a critical role in ensuring replication fidelity. In the non-segmented, positive-strand bat-borne RNA viruses like Severe Acute Respiratory Syndrome Coronaviruses (SARS-CoVs and MERS-CoV), it is found along with the methyltransferase (MTase) gene of the non-structural protein 14 (NSP14). In contrast, in the non-segmented, but negative-strand bat-borne RNA viruses such as Marburg, Ebola and Nipah viruses, it has been identified together with a novel type of capping enzyme, viz. guanosine diphosphate polyribonucleotidyltransferase (PRNTase). Hantaviruses, one of the fatal and emerging human pathogens in recent times, belong to segmented, negative-strand RNA viruses and are primarily rodent-borne. Currently, no universally approved vaccine or antiviral therapy exists for all hantavirus infections. The presence of a proofreading (PR) exonuclease domain has not been reported in hantaviruses until now. Interestingly, a putative PR exonuclease domain has been identified for the first time in these viruses within the large L protein, preceding the RNA-dependent RNA polymerase (RdRp). The newly identified domain belongs to the DEDD superfamily, but exhibits a distinct structural deviation, notably the replacement of the first catalytic triad -DxE- in the Old World hantaviruses with an -NxE- motif in most of the New World hantaviruses, and the use of an invariant tyrosine instead of histidine at the proton acceptor site. This feature closely resembles the DEDD superfamily of PR exonucleases reported in bacterial DNA polymerases I and II and in the DNA-dependent DNA polymerases of DNA viruses. To the best of my knowledge, this study provides the first evidence for the presence of a typical DEDD superfamily of PR exonuclease domain in hantaviruses, and thus, expanding our understanding of the structure and catalytic mechanism of 3′→5′ PR exonucleases in segmented, negative-strand RNA viruses and further presenting a novel structural target for the development of universally effective antiviral therapies against fatal hantavirus infections.