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. 2018 Mar 13;9:391. doi: 10.3389/fmicb.2018.00391

FIGURE 8.

FIGURE 8

Locations of the SV41 HN-derived amino acids in the SV41 HN-like chimera IM18-F. (A,B) Bottom (A) and side (B) views of IM18-F in the “2-heads-up/2-heads-down” conformation. The structures were drawn as ribbon models on the basis of the crystal structure (PDB ID: 4JF7) of the PIV5 (W3A) HN protein. The most amino-terminal residue in the model structure (ectodomain) is I62 in the stalk domain. The amino acid sequence identify between the ectodomains of IM18-F and the PIV5 HN protein is 47.3%. The head domains of the protomers which are in the down conformation are not shown. The positions of α-carbons of the SV41 HN-derived amino acids are indicated as green balls; in (A), only four amino acids (residue numbers in the parentheses) of the right protomer can be seem because the remaining five are hidden behind the stalk domain. FAR, F-activating region. (C) Bottom view of the head domain of IM18-F in the “4-heads-up” conformation. The structure was drawn as a ribbon model on the basis of the crystal structure (PDB ID: 1Z4Z) of the “receptor bound form” of PIV5 (W3A) HN head domain. The position of the stalk domain is predicted to be at the center of the tetramer and indicated as a black circle as in (A). The positions of α-carbons of the SV41 HN-derived amino acids are indicated as green balls. A black ball in each protomer indicates the α-carbons of the most amino-terminal residue L119 of the model structure. The amino acid sequence identify between the head domains of IM18-F and the PIV5 HN protein is 50.8%. (D) A hypothesis for the HN-F interaction. (a) Before receptor binding, the conformation of the HN-interacting region (HIR) in the F head domain is already modified if the F stalk domain has critical mutations. (b) After receptor binding, the HN protein undergoes a structural transition from the 4-heads-down conformation to the 4-heads-up conformation, which allows the FAR in the HN stalk domain to be exposed and to interact with the HIR in the F head domain. The conformation of the exposed FAR can be modified by critical mutations in the HN head domain.