TYROSINE PHOSPHORYLATION OF BLK AND FYN SRC HOMOLOGY-2 DOMAIN-BINDINGPROTEINS OCCURS IN RESPONSE TO ANTIGEN-RECEPTOR LIGATION IN B-CELLS AND CONSTITUTIVELY IN PRE-B CELLS

Proteins that bind to discrete domains of the Blk, Fyn, Lyn, and Btk p rotein tyrosine kinases were examined in pre-B cells that had not been subjected to any external stimulation, as well as in nonstimulated an d antigen-receptor-ligated B cells. Proteins that bind to the Src homo logy 2 domains of Blk and Fyn were identified in B cells that had been activated with anti-IgM but were not identified in unstimulated B tel ls. A number of Blk and Fyn Src homology 2 domain-binding phosphoprote ins were also observed in pre-B cells that had not been stimulated in vitro. The phosphoproteins seen in activated B cells potentially repre sent substrates that play a role in the pathway of antigen-receptor-me diated signaling. Distinct signaling pathways involving distinguishabl e kinase substrates may be relevant in pre-B-cell-receptor-mediated ce ll survival during ontogeny. These results indirectly support models t hat predict constitutive ligand-independent signaling by the pre-antig en receptor during lymphoid ontogeny.