Lck promotes Zap70-dependent LAT phosphorylation by bridging Zap70 to LAT.

Abstract

T cell antigen receptor (TCR) signaling requires the sequential activities of the kinases Lck and Zap70. Upon TCR stimulation, Lck phosphorylates the TCR, leading to the recruitment, phosphorylation, and activation of Zap70. Lck binds to and stabilizes phosho-Zap70 using its SH2 domain, and Zap70 phosphorylates the critical adaptors LAT and SLP76, which coordinate downstream signaling. It is unclear whether phosphorylation of these adaptors happens through passive diffusion or active recruitment. We report the discovery of a conserved proline-rich motif in LAT that mediated efficient LAT phosphorylation. Lck associated with this motif via its SH3 domain, and with phospho-Zap70 via its SH2 domain, thereby acting as molecular bridge to facilitate the co-localization of Zap70 and LAT. Elimination of this proline-rich motif compromised TCR signaling and T cell development. These results demonstrate the remarkable multi-functionality of Lck, where each of its domains has evolved to orchestrate a distinct step in TCR signaling.