FIG. 6.

Pax-5 and Id3 interact in vivo. (A) Coimmunoprecipitation (IP) of Id3 with Pax-5 from Cos-7 cells. Cells were transfected with expression vectors encoding Flag-tagged Pax-5 alone (lane 1), Id3 and Flag-tagged Pax-5 (lane 2), Id3 alone (lane 3), or Id3 and Flag-tagged Elk-1 (lane 4). (B) bHLH proteins and Pax proteins compete for Id binding. Coimmunoprecipitation of Id3 with Flag-tagged Pax-5 in the presence or absence of exogenous E47 (indicated above each lane). Flag-tagged proteins were immunoprecipitated, followed by detection of the Flag-tagged protein and interacting Id3 proteins by Western blotting with the indicated antibodies. The asterisk indicates a band that cross-reacts with the Flag epitope antibody. (C) Coimmunoprecipitation of endogenous Id3 and Pax-5 from Nalm-6 pre-B cells. Samples of total cell extract (from 10⁶ cells) and immunoprecipitates of Id3 complexes or of control serum (each from 10⁸ cells) were analyzed by Western blotting using Pax-5 antibody.