Ubiquitous endoprotease within constitutive secretory pathways capable of cleavage at the RX(K/R)R consensus motif (PubMed:11799113, PubMed:1629222, PubMed:1713771, PubMed:2251280, PubMed:24666235, PubMed:25974265, PubMed:7592877, PubMed:7690548, PubMed:9130696). Mediates processing of TGFB1, an essential step in TGF-beta-1 activation (PubMed:7737999). Converts through proteolytic cleavage the non-functional Brain natriuretic factor prohormone into its active hormone BNP(1-32) (PubMed:20489134, PubMed:21763278). By mediating processing of accessory subunit ATP6AP1/Ac45 of the V-ATPase, regulates the acidification of dense-core secretory granules in islets of Langerhans cells (By similarity). {ECO:0000250|UniProtKB:P23188, ECO:0000269|PubMed:11799113, ECO:0000269|PubMed:1629222, ECO:0000269|PubMed:1713771, ECO:0000269|PubMed:20489134, ECO:0000269|PubMed:21763278, ECO:0000269|PubMed:2251280, ECO:0000269|PubMed:24666235, ECO:0000269|PubMed:25974265, ECO:0000269|PubMed:7592877, ECO:0000269|PubMed:7690548, ECO:0000269|PubMed:7737999, ECO:0000269|PubMed:9130696}. (Microbial infection) Cleaves and activates diphtheria toxin DT. . (Microbial infection) Cleaves and activates anthrax toxin protective antigen (PA). {ECO:0000269|PubMed:1438214, ECO:0000269|PubMed:1644824}. (Microbial infection) Cleaves and activates HIV-1 virus Envelope glycoprotein gp160. . (Microbial infection) Required for H7N1 and H5N1 influenza virus infection probably by cleaving hemagglutinin. . (Microbial infection) Able to cleave S.pneumoniae serine-rich repeat protein PsrP. . (Microbial infection) Facilitates human coronaviruses EMC and SARS-CoV-2 infections by proteolytically cleaving the spike protein at the monobasic S1/S2 cleavage site. This cleavage is essential for spike protein-mediated cell-cell fusion and entry into human lung cells. . (Microbial infection) Facilitates mumps virus infection by proteolytically cleaving the viral fusion protein F. . This is the function of Ensembl gene identifier ENSG00000140564 (FURIN, furin, paired basic amino acid cleaving enzyme).