A PLC-γ1 Feedback Pathway Regulates Lck Substrate Phosphorylation at the T‑Cell Receptor and SLP-76 Complex

Phospholipase
C gamma 1 (PLC-γ1) occupies a critically important
position in the T-cell signaling pathway. While its functions as a
regulator of both Ca2+ signaling and PKC-family kinases
are well characterized, PLC-γ1’s role in the regulation
of early T-cell receptor signaling events is incompletely understood.
Activation of the T-cell receptor leads to the formation of a signalosome
complex between SLP-76, LAT, PLC-γ1, Itk, and Vav1. Recent studies
have revealed the existence of both positive and negative feedback
pathways from SLP-76 to the apical kinase in the pathway, Lck. To
determine if PLC-γ1 contributes to the regulation of these feedback
networks, we performed a quantitative phosphoproteomic analysis of
PLC-γ1-deficient T cells. These data revealed a previously unappreciated
role for PLC-γ1 in the positive regulation of Zap-70 and T-cell
receptor tyrosine phosphorylation. Conversely, PLC-γ1 negatively
regulated the phosphorylation of SLP-76-associated proteins, including
previously established Lck substrate phosphorylation sites within
this complex. While the positive and negative regulatory phosphorylation
sites on Lck were largely unchanged, Tyr192 phosphorylation
was elevated in Jgamma1. The data supports a model wherein Lck’s
targeting, but not its kinase activity, is altered by PLC-γ1,
possibly through Lck Tyr192 phosphorylation and increased
association of the kinase with protein scaffolds SLP-76 and TSAd.