Two-dimensional SDS-PAGE analysis of proteins associated with ΨL at the cell surface of the JEA2 proB-cell line. JEA2 cell surface labeling and anti-VpreB (4G7) or IgG1 control immunoprecipitation conditions are as described in Fig 6. Immunoprecipitates were run in the first and second dimensions under nonreducing and reducing conditions, respectively, using for both dimensions a 5% to 15% gradient SDS-PAGE. After transfer onto immobilon P membrane, biotinylated proteins were shown by streptavidin-peroxidase.