Association of CD19 and P-gp in Namalwa/MDR1 cells. (A) Coprecipitation of CD19 and P-gp in 1% Triton X-100 lysates of Namalwa/MDR1 cells. Equal amounts of the total cell lysate proteins were immunoprecipitated with the indicated MAbs and analyzed by Western blotting for the presence of P-gp, CD19, and CD22. (B-C) Flow cytometric analysis of FRET between PE anti–P-gp and Cy5 anti-CD19 or Cy5 anti-CD22 in Namalwa/MDR1 cells. Cells (10⁶) were incubated for 30 minutes on ice with 1 μg PE anti–P-gp, washed, and then incubated for another 30 minutes on ice in the absence or presence of 1 μg Cy5 anti-CD22 MAb or anti-CD19. The cells were washed, fixed with 1% paraformaldehyde, and analyzed on a FACS-Calibur flow cytometer. The bold line histogram represents background PE fluorescence in the absence of Cy5 anti-CD19 or anti-CD22, whereas filled histograms represent PE fluorescence in the presence of Cy5 anti-CD19 (B) or Cy5 anti-CD22 (C). PE fluorescence quenching in the presence of Cy5 anti-CD19 corresponds to the percent of energy transfer efficiency (E%) of 49.9%. Cy5 anti-CD22 did not induce fluorescence quenching (E = 1.1%). At least 3 independent experiments were performed. IP indicates immunoprecipitation.