1.G.3 The Viral Pore-forming Membrane Fusion Protein-3 (VMFP3) Family

The tick-borne encephalitis virus (TBEV) is a Class II fusion protein, consisting primarily of β-sheet structure with internal fusion peptides forming loops at the tips of β-strands. These proteins are in the virion, parallel to the viral membrane. There they exist as dimers that become β-stranded trimers upon membrane insertion and fusion (White et al. 2008). Fusion function depends on proteolytic processing. Other Class II VFPs include SFV E1/E2.

The flavivirus dengue virus (DV) infects cells through a low-pH-triggered membrane fusion reaction mediated by the viral envelope protein E. E is an elongated transmembrane protein with three C-terminal TMSs and is organized as a homodimer on the mature virus particle. During fusion, the E protein homodimer dissociates, inserts the hydrophobic fusion loop into target membranes, and refolds into a trimeric hairpin in which domain III (DIII) packs against the central trimer. It is clear that E refolding drives membrane fusion. Liao et al. (2010) used truncated forms of the DV E protein to reconstitute trimerization in vitro. Protein constructs containing domains I and II (DI/II) were monomeric and interacted with membranes to form core trimers. DI/II-membrane interaction and trimerization occurred efficiently at both neutral and low pH. The DI/II core trimer was relatively unstable and could be stabilized by binding exogenous DIII or by the formation of mixed trimers containing DI/II plus E protein with all three TMSs. The mixed trimer has unoccupied DIII interaction sites that specifically bind exogenous DIII at either low or neutral pH. Protein constructs containing domains I and II (DI/II) were monomeric and interacted with membranes to form core trimers. DI/II-membrane interaction and trimerization occurred efficiently at both neutral and low pH. The DI/II core trimer was relatively unstable and could be stabilized by binding exogenous DIII or by the formation of mixed trimers containing DI/II plus E protein with all three TMSs. The mixed trimer has unoccupied DIII interaction sites that specifically bind exogenous DIII at either low or neutral pH. Truncated DV E proteins thus reconstitute hairpin formation and define properties of key domain interactions during DV fusion.