The binding of the membrane proximal domain to viral HA is sialic acid dependent. NKp46D1-Ig (A,C) or NKp46D2-Ig (B,D) cells were treated with NA (▪) or with PBS (as control; □). SV-(A-B) or IV-(C-D) infected and uninfected 721.221 or 1106mel cells, respectively, were washed and stained with either the PBS-treated or NA-treated fusion proteins followed by PE-conjugated goat antihuman Fc antibodies. Figure shows one representative experiment of 3 performed.