Comparisons of Belmont Virus, a Possible Bunyavirus Unique to Australia, with Bunyamwera Virus

Abstract

Belmont virus is an arbovirus isolated from mosquitoes and has a preference for marsupial hosts. The diameter of virions by negative staining (122 nm before fixation and 91 nm after fixation) was greater than that of Bunyamwera virus (94 nm and 79 nm respectively). However, the particles of both viruses appeared morphologically identical and sedimented at the same rate in sucrose density gradients. Belmont virus had a tripartite segmented RNA genome (28S, 24S and 11S) similar to Bunyamwera virus RNA (33S, 26S and 16S). The mol. wt. of these RNA species of Belmont virus measured by gel electrophoresis was 3.2 x 10(6), 2.4 x 10(6) and 0.3 x 10(6) compared to 2.9 x 10(6), 1.8 x 10(6) and 0.3 x 10(6) for the L, M and S species of Bunyamwera virus RNA. Both viruses comprised four structural proteins of the same relative proportions and corresponding mol. wt. For Bunyamwera virus, these were 145 x 10(3) (L), 104 x 10(3) (G1), 32 x 10(3) (G2) and 22 x 10(3) (N). The equivalent proteins of Belmont virus had mol. wt. of 147 x 10(3) (P147), 107 x 10(3) (G107), 28 x 10(3) (P28) and 25 x 10(3) (P25). Under conditions in which the envelope glycoproteins G1 and G2 of Bunyamwera virus were labelled in glucosamine, only G107 of Belmont virus was labelled. However, both G107 and P28 of Belmont virus were solubilized by non-ionic detergent and were then separable from the nucleocapsid containing all the RNA and P25. Chymotrypsin treatment of Belmont virus digested only G107, leaving a residue of P25 and P28, and of visible spikes. Similarly, G2 and the spikes of Bunyamwera virus resisted digestion with chymotrypsin. It was concluded that P28 is an envelope protein, equivalent to G2. Belmont virus thus appears to be a typical member of the Bunyaviridae but is unique in that it lacks carbohydrate in the small envelope protein (P28).