Persistent reovirus infections of L cells select mutations in viral attachment protein sigma 1 that alter oligomer stability

During maintenance of L-cell cultures persistently infected with reovirus, mutations are selected in viruses and cells. Cells cured of persistent infection support growth of viruses isolated from persistently infected cultures (PI viruses) significantly better than that of wild-type (wt) viruses, In a previous study, the capacity of PI virus strain WC to grow better than wt strain type 1 Lang (T1L) in cured cells was mapped genetically to the S1 gene (R. S. Kauffman, R. Ahmed, and B. N. Fields, Virology 131:79-87, 1983), which encodes viral attachment protein sigma 1. To investigate mechanisms by: which mutations in S1 confer growth of PI viruses in cured cells, we determined the S1 gene nucleotide sequences of WC virus and six additional PI viruses isolated from independent persistently infected L-cell cultures. The S1 sequences of these viruses contained from one to three mutations, and with the exception of Pf 2A1, mutations in each S1 gene resulted in changes in the deduced amino acid sequence of sigma 1 protein, Using electrophoresis conditions that favor migration of sigma 1 oligomers, we found that sigma 1 proteins of L/C, PI 1A1, PI 3-1, and PI 5-1 migrated as monomers, whereas sigma 1 proteins of wt reovirus and PI 2A1 migrated as oligomers, These findings suggest that mutations in sigma 1 protein affecting stability of sigma 1 oligomers are important for the capacity of PI viruses to infect mutant cells selected during persistent infection, Since no mutation was found in the deduced amino acid sequence of PE 2A1 sigma 1 protein, we used T1L x PI 2A1 reassortant viruses to identify viral genes associated with the capacity of this PI virus to grow better than wt in cured cells. The capacity of PI 2A1 to grow better than T1L in cured cells was mapped to the S4 gene, which encodes outer-capsid protein sigma 3. This finding suggests that in some crises, mutations in sigma 3 protein in the absence of al mutations confer growth of PI viruses in mutant cells, To confirm the importance of the S1 gene in PI virus growth in cured cells, we used T1L x PI 3-1 reassortant viruses to genetically map the capacity of this PI virus to grow better than wt in cured cells, In contrast to our results using PI 2A1, we found that growth of PI 3-1 in cured cells was determined by the sigma 1-encoding S1 gene, Given that the sigma 1 and sigma 3 proteins play important roles in reovirus disassembly, findings made in this study suggest that stability of the viral outer capsid is an important determinant of the capacity of reoviruses to adapt to host cells during persistent infection.