Mutagenesis of the RGD motif in the yellow fever virus 17D envelope protein

The envelope protein of yellow fever virus 17D (YFV-17D) contains a solvent-exposed RGD motif, which has led to the suggestion that integrins may function as cellular receptors for YFV-17D. We found that mutating the RGD motif to RGE had no effect on viral titers, whereas changing RGD to TGD, TGE, TAD, TAE, or RGS led to reduced titers. Substitution of RGD by RAD or RAE yielded RNA genomes that replicated in mammalian cells but could not spread to neighboring cells at 37 degrees C. These mutants did spread through the cell monolayer at 30 degrees C (both in mosquito cells and in SW13 cells) and viruses grown at this temperature were capable of infecting mammalian cells at 37 degrees C. These results strongly suggest that RGD-mediated integrin binding does not play a major role in YFV-17D entry, since the RGD to RAD mutation, as well as many or all of the other mutations studied, should disrupt all RGD-dependent integrin binding. However, the RGD to RAD or RAE mutations (as well as TAD and TAE) severely destabilized the envelope protein at 37 degrees C, providing an explanation for the observed phenotype. Implications of these findings are discussed in light of the fact that mutations that alter tropism or virulence in different flaviviruses are often found within the loop containing the RGD motif.