FIGURE 6.
Tolerance of the V1, V4, and V5 loops for foreign protein insertion. A, amino acid sequence alignment among GFP variants. Only the alignments around the nonidentical amino acid residues (boldface) are shown. The numbers above the alignments are from GFPOPT. The sfGFP used in this study carried a deletion of 8 amino acid residues at the C terminus (indicated as gray letters). B and C, several foreign proteins were inserted into the V4, V5 (B), or V1 (C) loop of gp120. The relative cell-cell fusion activities measured by the DSP assay, cell surface expression levels measured by the CELISA, and normalized cell-cell fusion activities are shown by white, gray, and black bars, respectively. Clover-GIT had an additional Gly-Ile-Thr sequence at the C terminus. Data represent the means ± S.D. for at least seven wells from two independent experiments. Error bars represent S.D.