BLNK is associated with the CD72/SHP-1/Grb2 complex in the WEHI231 cell line after membrane IgM cross-linking

Tyrosine phosphorylation of CD72 strongly correlates with B cell antigen re ceptor signals leading to apoptosis. We have previously shown that CD72 car rying two immunoreceptor tyrosine-based inhibition motifs (ITIM) is an in v ivo substrate of SHP-1. CD72 forms a complex with SHP-1 and Grb2 via its ty rosine-phosphorylated ITIM when the WEHI231 cell line, which is representat ive of immature B cells, undergoes apoptosis. The CD72 complex formation wa s also demonstrated in normal primary B cells, suggesting that the complex formation in apoptotic B cells is a universal mechanism. In this study, we further investigated the molecular components of the CD72 complex in WEHI23 1 cells in order to understand the molecular mechanism involved in the sign aling pathway mediated through the complex. Our experiments demonstrate tha t BLNK, a recently identified adaptor molecule predominantly expressed in B cells, is associated with the CD72 complex via the Src homology 3 domain(s ) of Grb2 in the cell line after membrane IgM (mIgM) engagement. The result s suggest that the mIgM-mediated signal strongly correlates with the format ion of the CD72/SHP-1/Grb2/BLNK complex.