Fig. S1.

Antigen quality assessment by SDS/PAGE. Composite image of reducing SDS/PAGE gels shows samples of each recombinant ectodomain antigen (1 µg, determined by absorbance at 285 nm), after nickel column purification, arranged in order of descending predicted molecular weight. In most cases, the low expression level of the antigen necessitated processing of large volumes of tissue culture supernatant; this resulted in the retention of nonspecfic proteins, such as serum albumin. The presence of the desired antigen in each purified sample was determined by ELISA using tag-specific antibodies before immunization. M, protein markers.