FIG. 7.

Domain exchange could have evolved with only a few somatic mutations. Extensive site-directed mutagenesis was performed, and each substitution was analyzed for domain exchange. The percentage of domain exchange was graphed versus the number of germ line residues of the 2G12 heavy chain that were changed into wild-type ones (i.e., 0 equals germ line, and 38 equals wild-type including all somatic hypermutations). About five residues are necessary to get a fraction of domain exchange significantly higher than a linear trend (shaded area). The variants gl2G12dx, gl2G12dxb, gl2G12dxbi, and gl2G12dxEA are depicted as open circles. The panels of individual germ line reversion in the gl2G12dxEA (Fig. 3A) and wt2G12 (Fig. 3B) backgrounds are shown with upright and inverted triangles, respectively. A potential pathway for evolution into a domain-exchanged antibody is shown (squares). Diamonds show other ineffective and unrelated variants that were tested (see Table S1 in supplemental material elsewhere [http://www.scripps.edu/ims/burton/supplemental/Huber_JVI_2010.pdf]).