Avian reovirus sigma C protein contains a putative fusion sequence and induces fusion when expressed in mammalian cells.

The biological functions of the structural protein sigma C, from avian reovirus strain RAM-1, were investigated in this study. A putative fusion peptide in sigma C was recognized in the deduced amino acid sequence by homology with Pneumovirus fusion sequences, and it was thus postulated that this protein may be involved in the formation of syncytia in cells infected with RAM-1. The sigma C gene was cloned and expressed in mammalian (COS7) cells and the sigma C protein was found to induce syncytia. It was therefore concluded that this protein is indeed responsible for avian reovirus-induced cell fusion. It was also found that sigma C caused condensation of the nuclei within a syncytium, as observed in RAM-1-infected cells. On the basis that this represented condensation of the chromatin, the inhibition of cellular DNA synthesis by the virus and by the sigma C protein was measured. It was found that the virus caused a 50% reduction in cellular DNA synthesis, but the sigma C protein did not inhibit DNA synthesis. Therefore pyknosis of the nuclei and inhibition of cellular DNA synthesis by RAM-1 are likely to be separate events.