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Cooperation Amongst Distinct RNA Virus Genomes Enhance Syncytium Formation (Cell fusion)

Measles virions have two envelope glycoproteins, a receptor binding hemagluttinin (H) and a fusion protein (F). When H binds to its cellular receptor, it is thought that the F protein undergoes conformational changes that assist in virus-cell fusion as well as cell-to-cell fusion. The ladder also called syncytium formation or syncytia, are multinucleated cells that aid the virus in the later stages of infection where they can pass from cell to cell without being extracellularly exposed to the immune system.
This paper initially identified two genotypes through overlapping peaks in Sanger sequence data, implying distinct sequences, and therefore distinct genomes. Since these mutations changed the amino acid sequences of the Fusion protein, they analyzed these samples for their syncytium forming abilities. By coexpressing two plasmids with each type of F sequence (F1 and F2), they found that when expressed together, F1 and F2 displayed enhanced fusion activity compared to either by themselves, including the wild type sequence.
In animal studies, they monitored the spread of these two distinct genotypes in hamster brains, which do not express Measles virus receptors. They found that this cooperation amongst distinct genomes provided the virus with the ability to spread more efficiently in this 'receptor-negative' tissue, where the wild type virus does not propagate.
The proposed model is that since the F protein exists as a heterotrimeric protein (3 protein monomers) these two distinct F genotypes recombine at the protein level at a specific ratio to form a novel F protein. This provides these new viruses with an enhanced ability to spread intracellularly and therefore outside the detection limits of antibodies. This only occurs when these two genomes are expressed in the same cell demonstrating a remarkable cooperative adaptation for RNA viruses.

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