A number of earlier studies of three plaque variants of Mengo virus (designated L-, M-, and S-Mengo), all of which suggested that they differ from one another with respect to surface charge characteristics, prompted an examination of the variants by the technique of isoelectric focusing. Highly purified, radiolabelled virus preparations were subjected to electrofocusing in sucrose-stabilized, pH gradients in the presence of 0.5% Brij 35. The distribution of each variant was found to be bimodal; the average values for the pH's at which the major (75–90% of input virions) and minor peaks were found were as follows: L-Mengo, 8.1 and 4.6; M-Mengo, 4.4 and 6.3; S-Mengo, 4.6 and 6.8 Infectivity assays established that the distributions of plaque-forming units, (p.f.u.) and radioactivity were the same (c.p.m./p.f.u. ratios in the major and minor peaks were essentially identical), and re-electrofocusing of virions from each peak was found to give the same bimodal distribution as did the original preparation. The M- and S- variants were found to migrate more rapidly than L-Mengo during electrophoresis in polyacrylamide gels at pH 8.3. The data arising from the electrofocusing studies are compatible with observations regarding the relative rates of attachment of L-, M-, and S-Mengo to cultured L cells and their relative abilities to hemagglutinate human erythrocytes (type O) at pH 8.0.
