This study was supported by Wellcome Trust Grant 099220/B/12/Z (to R.M.E.) and Grant 094476/Z/10/Z that funded the purchase of the TripleTOF 5600 mass spectrometer at the Biomedical Sciences Research Complex (BSRC) of University of St. Andrews. ; Bunyamwera virus (BUNV) is the prototype of the Orthobunyavirus genus and Bunyaviridae family that contains important human and animal pathogens. The cleavage mechanism of orthobunyavirus glycoprotein precursor (GPC) and the host proteases involved have not been clarified. Here we found that NSm and Gc contain their own internal signal peptides, which mediate the GPC cleavage by host signal peptidase and signal peptide peptidase (SPP). Furthermore, the NSm domain-I plays an important postcleavage role in cell fusion. Our data clarified the implication of host proteases in the processing of the orthobunyavirus GPC. This work identifies SPP as a potential intervention target, and the knowledge we gained will benefit preventive strategies against other orthobunyavirus infections. ; Postprint ; Peer reviewed