Furin-mediated protein processing in infectious diseases and cancer

Braun, Elisabeth; Sauter, Daniel

peptide. 63, 64 MARV and human pathogenic Ebolavirus species harbour canonical furin cleavage sites (R-X-K/R-R↓). In contrast, the GP of the closely related Reston virus, which causes asymptomatic infections in humans, is processed less efficiently by furin as it carries the suboptimal cleavage site K-Q-K-R↓. 63 Surprisingly, however, uncleavable GP mutants of highly pathogenic Ebola virus (EBOV) are able to mediate infection and furin-mediated cleavage is not required for replication in cell culture. 65 Furthermore, an EBOV mutant lacking the furin cleavage site replicated efficiently in nonhuman primates and showed no differences in disease progression or lethality compared to wild-type viruses. 66 Thus, the high conservation of the furin cleavage site among different Ebolavirus species is surprising and it remains to be determined whether furin-mediated GP processing plays a role in the natural reservoir hosts of these viruses. 65 Notably, the EBOV GP gene harbours an RNA editing site sequence and may not only express fulllength GP, but also a soluble form of the glycoprotein (pre-sGP) that lacks the C-terminal transmembrane domain. 67 Intriguingly, pre-sGP harbours another furin recognition site and is cleaved into mature sGP and a short so-called D peptide. Both of them are ultimately released from infected cells. 68 Although pre-sGP is produced in higher amounts than GP, its role in viral replication is under debate. Among others, sGP has been suggested to serve as a decoy antigen, to act as a structural substitute for GP1 and to induce apoptosis of uninfected lymphocytes. 69 Flaviviruses Flavivirus RNA is translated into a single large polyprotein that is cleaved by cellular and viral proteases into all structural and nonstructural proteins of the virus. The structural proteins comprise the envelope proteins prM and E that are incorporated as prM/E heterodimers into budding virions. 70 prM acts as a chaperone and facilitates correct folding of the E glycoprotein. 71 Many flaviviruses bud into the lumen of the ER and enter the secretory pathway. 70 In the acidic milieu of the trans-Golgi network (TGN), a furin cleavage site is exposed and prM can be cleaved into mature pr and M proteins. 72 Thus, furinmediated cleavage of the viral glycoprotein occurs only after its incorporation into newly formed virions (Figure 3a, right panel) . This is in contrast to viruses such as HIV, whose envelope proteins are cleaved before assembly. The pr peptide remains associated with the E protein until the virion is released from the cell, thereby preventing premature unintended fusion with membranes of the producer cell. 73 Furin-mediated prM cleavage is essential for replication of flaviviruses such as tick-borne encephalitis or dengue virus. 74, 75 In some cases, prM molecules escape furin processing in the producer cell and result in the release of immature or partially mature viral particles. Partially mature virions are still infectious since low amounts of mature M are sufficient to mediate fusion with target cell membranes. 70 Furthermore, uncleaved prM may participate in virion attachment to target cells and can be cleaved by furin during the entry process, in the acidic milieu of endosomes 76, 77 (Figure 3b , left panel). The relative contribution of prM processing during viral entry into new target cells, however, remains to be determined.