Domain-III FG loop of the dengue virus type 2 envelope protein is important for infection of mammalian cells and Aedes aegypti mosquitoes

The FG extended loop in domain III of the dengue virus type 2 (DENV2) envelope protein is postulated to be a molecular determinant for host cell infectivity. To determine the contribution of the FG loop to virus infectivity, an infectious cDNA clone of DENV2 was manipulated by deleting amino acids in the loop (VEPGΔ) to mimic tick-borne flaviviruses or by substituting these AAs with RGD or RGDK/S to mimic motifs present in other mosquito-borne flaviviruses. We found the FG loop to be dispensable for infection of C6/36 cells but critical for infection of Aedes aegypti mosquito midguts and mammalian cells. All the FG loop mutants were able to bind to and enter mammalian cells but replication of VEPGΔ in Vero cells at 37 °C was delayed until acquisition of secondary mutations. Reduced binding of DENV2 type-specific monoclonal antibody 3H5 to mutant viruses confirmed the FG loop motif as its target epitope.