Monoclonal antibodies as structural probes of southern bean mosaic virus

The reactivity of six monoclonal antibodies with native southern bean mosaic virus particles, swollen virus, swollen virus contracted by divalent cations or by pH adjustment to 5.0, and virus coat protein at pH 7.5 and 5.0 were assessed by antigen inhibition enzyme-linked immunosorbent assays. Clones B4, B7, and B11 did not react with native virus but were inhibited by low levels of all nonnative virus antigens. B6 had a high reactivity only with the native virus. B5 and B10 reacted equally with native virus and swollen virus contracted by Ca2+ or by pH adjustment, but reacted only weakly with swollen virus or swollen virus contracted by Mg2+. B5 reacted with viral protein at pH 5.0 and weakly with coat protein at pH 7.5, whereas B10 did not react with protein at pH 7.5 and had only limited reactivity with viral protein at pH 5.0. The reactivity of B5 and B10 with swollen virus increased as the pH was gradually decreased between 7.3 and 6.5. Similarly, their reactivity with swollen-contracted virus decreased as the pH was increased within the same range. Reactivity versus pH plots of these two transition forms showed hysteresis with B10 but did not with B5.