Cbl-b, a mammalian homolog of Cbl, consists of an N-terminal region (Cbl-b-
N) highly homologous to oncogenic v-Cbl, a Ring finger, and a C-terminal re
gion containing multiple proline-rich stretches and potential tyrosine phos
phorylation sites. In the present study, we demonstrate that upon engagemen
t of the T cell receptor (TCR), endogenous Cbl-b becomes rapidly tyrosine-p
hosphorylated, In heterogeneous COS-1 cells, Cbl-b was phosphorylated on ty
rosine residues by both Syk(Syk/Zap-70) and Src- (Fyn/Lck) family kinases,
with Syk kinase inducing the most prominent effect. Syk associates and phos
phorylates Cbl-b in Jurkat T cells, A Tyr-316 Chi-binding site in Syk was r
equired for the association with and for the maximal tyrosine phosphorylati
on of Cbl-b, Mutation at a loss-of-function site (Gly-298) in Cbl-b-N disru
pts its interaction with Syk, Cbl-b constitutively binds Grb2 and becomes a
ssociated with Crk-L upon TCR stimulation. The Grb2- and the Crk-L-binding
regions were mapped to the C-terminus of Cbl-b, The Crk-L-binding sites wer
e further determined to be (YDVP)-D-655 and (YKIP)-K-709, With th, latter b
eing the primary binding site. Taken together, these results implicate that
Cbl-b is involved in TCR-mediated intracellular signaling pathways.