doi:10.1099/0022-1317-64-7-1457. total of four amino acid variants (leucine [L], proline [P], serine [S], and threonine [T]). We demonstrate that P8 affects the proteolytic processing and the fusogenicity of MuV F. The presence of L or S at P8 resulted in a slower proteolysis of MuV F by furin and a reduced ability to mediate cell-cell fusion. However, virus-cell fusion was more efficient for F proteins harboring L or S at P8, suggesting that P8 contributes to the mechanism of viral spread: P and T enable a rapid spread of illness by cell-to-cell fusion, whereas viruses harboring L or S at P8 spread preferentially from the launch of infectious viral particles. Our study provides novel insights into the fusogenicity of MuV and its influence within the mechanisms of virus spread within infected tissues. Presuming a correlation between MuV fusogenicity and virulence, sequence information within the amino acid residue at P8 might be helpful to estimate the virulence of circulating and growing strains. IMPORTANCE Mumps computer virus (MuV) is the causative agent AM 103 of the highly infectious disease mumps. Mumps is mainly associated with slight symptoms, but severe complications such as encephalitis, meningitis, or orchitis can also happen. There is evidence the virulence of different MuV strains and variants might correlate with the ability of the fusion protein (F) to mediate cell-to-cell fusion. However, the connection between virulence and fusogenicity or the mechanisms responsible for the varied fusogenicity of different MuV strains are incompletely recognized. Here, we focused on the amino acid residue at position 8 (P8) of the proteolytic cleavage site of MuV F, because this amino acid residue shows a impressive variability depending on the genotype of MuV. The P8 residue has a significant effect on NES the proteolytic processing and fusogenicity of MuV F and might therefore determine the route of viral spread within infected tissues. within the family test with 95% confidence intervals. *, test with 95% confidence intervals. *, test with 95% confidence intervals. *, test with 95% confidence intervals. *, test with 95% confidence intervals. *, toxin, toxin, particular hemagglutinins of highly pathogenic avian influenza viruses, Dengue computer virus prM, and human being papillomavirus small capsid protein L2 (15, 46,C51). The cleavage of MuV F by furin primarily happens during its maturation in the TGN within the infected cell (12,C15). For MuV F P8-P and P8-T, fast proteolytic cleavage might result in the manifestation of cleaved F proteins on AM 103 the surface of transfected or infected cells, therefore enabling a direct fusion with neighboring cells. Vice versa, the slower and therefore less efficient intracellular cleavage of MuV F P8-L and P8-S prospects to a larger proportion of uncleaved F proteins within the cell surface, prohibiting the subsequent cell-to-cell fusion. Given that these viruses are still able to mediate fusion with the plasma membrane, it is very likely that uncleaved MuV F P8-L and P8-S proteins are proteolytically triggered by cell surface furin during the access process into fresh target cells. As a result, the proteolytic activation within the cell surface allows F-mediated fusion between viral and cellular membranes and spread of illness without inducing severe cytopathic effects such as the formation of syncytia. However, the cleavage of MuV F P8-L and P8-S on the surface of transfected cells does not allow the induction of F-mediated cell-to-cell fusion comparable to that of MuV F P8-P and P8-T. Further, viral entryspecifically, the fusion between viral and sponsor cell membranesis more efficient for F P8-L or AM 103 P8-S, suggesting that cell-to-cell and virus-cell fusion might require different conditions. The presence of L or S at P8 might provide conditions that are beneficial for virus-cell fusion, whereas for P or T, cell-to-cell fusion is definitely more efficient. This assumption is definitely encouraged from the observation that MuVs harboring P8-L or P8-S yield higher titers of cell-free computer virus than intracellular computer virus. It was previously demonstrated for a number of viruses, including mouse hepatitis coronavirus, Sendai computer virus, herpesviruses, and human being immunodeficiency computer virus type 1, that virus-induced fusion between neighboring cells or between viruses and cells seems to symbolize two unequal processes with different requirements, even though underlaying mechanisms are not fully understood (52,C57). In sum, our findings demonstrate that one single amino acid residue close to the cleavage motif of MuV F affects the proteolytic processing as well as the degree and type of.