Physical and functional interactions between Lyn and p34(cdc2) kinases in irradiated human B-cell precursors

Exposure of human B-cell precursors (BCP) to ionizing radiation results in cell cycle arrest at the G(2)-M checkpoint as a result of inhibitory tyrosine phosphorylation of p34(cdc2). Here, we show that ionizing radiation promotes physical interactions between p34(cdc2) and the Src family protein-tyrosine kinase Lyn in the cytoplasm of human BCP leading to tyrosime phosphorylation of p34(cdc2). 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(cdc2) on tyrosine 15. Furthermore, Lyn kinase physically associated with and tyrosine-phosphorylated p34(cdc2) kinase in vivo when coexpressed 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(cdc2) 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(cdc2) from BCP lysates. Ionizing radiation failed to cause tyrosine phosphorylation of p34(cdc2) or G(2) arrest in Lyn kinase-deficient BCP, supporting am important role of Lyn kinase in radiation-induced G(2) phase-specific cell cycle arrest. Our findings implicate Lyn as an important cytoplasmic suppressor of p34(cdc2) function.