Exposure of human B-cell precursors (BCP) to ionizing radiation results in cell cycle arrest at the G-M checkpoint as a result of inhibitory tyrosine phosphorylation of p34. Here, we show that ionizing radiation promotes physical interactions between p34 and the Src family protein-tyrosine kinase Lyn in the cytoplasm of human BCP leading to tyrosine phosphorylation of p34. Lyn kinase immunoprecipitated from lysates of irradiated BCP as well as a full-length glutathione S-transferase (GST)-Lyn fusion protein-phosphorylated recombinant human p34 on tyrosine 15. Furthermore, Lyn kinase physically associated with and tyrosine-phosphorylated p34 kinase in vivo when co-expressed in COS-7 cells. Binding experiments with truncated GST-Lyn fusion proteins suggested a functional role for the SH3 rather than the SH2 domain of Lyn in Lyn-p34 interactions in BCP. The first 27 residues of the unique amino-terminal domain of Lyn were also essential for the ability of GST-Lyn fusion proteins to bind to p34 from BCP lysates. Ionizing radiation failed to cause tyrosine phosphorylation of p34 or G arrest in Lyn kinase-deficient BCP, supporting an important role of Lyn kinase in radiation-induced G phase-specific cell cycle arrest. Our findings implicate Lyn as an important cytoplasmic suppressor of p34 function.

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