FIG. 1.

The leucine zipper domain of the MHV-A59 S protein. (A) Schematic diagram of MHV-A59 S2 subunit showing the relative location of the leucine zipper domain in relation to other distinctive domains in S2. The thin black line represents the entire S2 region. Distinctive domains are shown as rectangles with their names above. LZM, leucine zipper domain (4); HR1 and HR2, heptad repeat regions 1 and 2 (14); TM, transmembrane domain (30); PEP1, fusion peptide-like region (28). The region representing PEP1 is hatched since it is part of HR1. LZM and HR2 represent the same region of MHV-A59 S. (B) Amino acid sequence of the leucine zipper domain of wild-type and mutant S proteins. The heptadic leucine and isoleucine residues are boxed, and their corresponding positions are marked above in reference to the starting methionine residue of the S protein. The leucine zipper domain sequences of single- or double-alanine-substitution mutants are listed below the wild-type sequence. Leucine- or isoleucine-to-alanine substitutions in those mutants are indicated by the letter A, while the dashed lines represent sequences identical to that of the wild-type S. aa, amino acids.