In 2017, the order Mononegavirales was expanded by the inclusion of a total of 69 novel species. Five new rhabdovirus genera and one new nyamivirus genus were established to harbor 41 of these species, whereas the remaining new species were assigned to already established genera. Furthermore, non-Latinized binomial species names replaced all paramyxovirus and pneumovirus species names, thereby accomplishing application of binomial species names throughout the entire order. This article presents the updated taxonomy of the order Mononegavirales as now accepted by the International Committee on Taxonomy of Viruses (ICTV).

In 2018, the order Mononegavirales was expanded by inclusion of 1 new genus and 12 novel species. This article presents the updated taxonomy of the order Mononegavirales as now accepted by the International Committee on Taxonomy of Viruses (ICTV) and summarizes additional taxonomic proposals that may affect the order in the near future.

Nipah virus (NiV) is a recently emerged and highly pathogenic paramyxovirus that causes a systemic infection in animals and humans and can infect a wide range of cultured cells. Interestingly, the NiV fusion (F) protein has a single arginine at the cleavage site similar to paramyxoviruses that are activated by exogenous trypsin-like enzymes only present in specific cells and tissues and therefore only cause localized infections. We show here that NiV F activation is not mediated by an exogenous serum protease but by an endogenous ubiquitous cellular protease after endocytosis of the protein. In addition to endocytosis, acidification of the endosome is a prerequisite for F cleavage. These results show that activation of the NiV F protein depends on a type of proteolytic cleavage that is clearly different from what is known for other paramyxoviral and orthomyxoviral fusion proteins. To our knowledge, this is the first example of a viral class I fusion protein whose activation depends on clathrin-mediated constitutive endocytosis.

Proteolytic activation of the fusion protein of the highly pathogenic Nipah virus (NiV F) is a prerequisite for the production of infectious particles and for virus spread via cell-to-cell fusion. Unlike other paramyxoviral fusion proteins, functional NiV F activation requires endocytosis and pH-dependent cleavage at a monobasic cleavage site by endosomal proteases. Using prototype Vero cells, cathepsin L was previously identified to be a cleavage enzyme. Compared to Vero cells, MDCK cells showed substantially higher F cleavage rates in both NiV-infected and NiV F-transfected cells. Surprisingly, this could not be explained either by an increased F endocytosis rate or by elevated cathepsin L activities. On the contrary, MDCK cells did not display any detectable cathepsin L activity. Though we could confirm cathepsin L to be responsible for F activation in Vero cells, inhibitor studies revealed that in MDCK cells, cathepsin B was required for F-protein cleavage and productive replication of pathogenic NiV. Supporting the idea of an efficient F cleavage in early and recycling endosomes of MDCK cells, endocytosed F proteins and cathepsin B colocalized markedly with the endosomal marker proteins early endosomal antigen 1 (EEA-1), Rab4, and Rab11, while NiV F trafficking through late endosomal compartments was not needed for F activation. In summary, this study shows for the first time that endosomal cathepsin B can play a functional role in the activation of highly pathogenic NiV.

Nipah virus (NiV), a highly pathogenic paramyxovirus, causes a systemic infection in vivo and is able to replicate in cultured cells of many species and organs. Such pantropic paramyxoviruses generally encode fusion (F) proteins with multibasic cleavage sites activated by furin or other ubiquitous intracellular host cell proteases. In contrast, NiV has an F protein with a single arginine (R ₁₀₉ ) at the cleavage site, as is the case with paramyxoviruses that are activated by trypsin-like proteases only present in specific cells or tissues and therefore only cause localized infections. Unlike these viruses, cleavage of the NiV F protein is ubiquitous and does not require the addition of exogenous proteases in cell culture. To determine the importance of the amino acid sequence at the NiV F protein cleavage site for ubiquitous activation, we generated NiV F proteins with mutations around R ₁₀₉ . Surprisingly, neither the exchange of amino acids upstream of R ₁₀₉ nor replacement of the basic residue itself interfered with F cleavage. Thus, R ₁₀₉ is not essential for F cleavage and activation. Our data demonstrate that NiV F-protein activation depends on a novel type of proteolytic cleavage that has not yet been described for any other paramyxovirus F protein. NiV F activation is mediated by a ubiquitous protease that requires neither a monobasic nor a multibasic cleavage site and therefore differs from the furin- or trypsin-like proteases known to activate other ortho- and paramyxovirus fusion proteins.