FIG. 2.

Identification of domains in the middle region of the PIV5 F protein that are exposed on the trimer surface. (A) Schematic diagram of the PIV5 F protein. Positions of four major domains (M1, M2, M3, and M4) in the middle region are indicated. FP, fusion peptide; TM, transmembrane domain; HRA and HRB, heptad repeat regions A and B. (B) Amino acid sequence alignment of the middle regions of the F proteins of PIV5 and SV41. Positions of the four major domains and five minor domains (A to E) are indicated. Amino acid residues of the PIV5 F protein that are exposed on the trimer surface are underlined. Dashes in the SV41 F sequence indicate amino acids identical to those of the PIV5 F protein. (C) Location of the middle region and exposed domains on the 3-D structures of the PIV5 F trimer. For simplicity, the major and minor domains of only one monomer are shown, as capital letters; some of the minor domains of the remaining two monomers are shown by small letters. The positions of Ala-47, Thr-97, and Ala-111, which correspond to Arg-164, Gly-219, and Val-233 of the F protein of CDV strain Onderstepoort, respectively, are indicated in the side view of the PIV5 F trimer. The figures were depicted with the aid of the DeepView Swiss-PdbViewer program on the basis of the crystal structure of the PIV5 F cleavage site mutant.