Homodimeric association of the spike glycoproteins G1 and G2 of Uukuniemi virus.

We have studied the interactions of the G1 and G2 membrane glycoproteins of Uukuniemi virus, a bunyavirus, in virus particles and in Triton X-100-solubilized virus. The G1 glycoprotein in intact virus or in Triton solution could be oxidized into a covalent homodimer using Cu2+ ion as a catalyst. Immunoprecipitations of the glycoproteins from Triton-solubilized virus lysates showed that G1 and G2 do not form a stable heterodimeric or heterooligomeric complex. The oligomeric association of G1 and G2 was further analyzed using centrifugation in sucrose gradients in the presence of Triton X-100. The results indicate that G1 exists as a Triton-resistant pH-insensitive homodimer. This is in contrast to the behavior of G2, which exists as a homodimer and partially as a monomer at pH 6.4 or above and is dissociated completely into a monomer at pH 6.0 or below. The threshold for the dimer-monomer shift of G2 is between pH 6.2 and pH 6.0. Electron microscopy studies show that the surface structure of the virus particle undergoes a pH-dependent change. Studies on the kinetics of virus entry suggest that pH below 6.2 is necessary for the penetration of Uukuniemi virus.