Extended Data Fig. 10. Proposed Model.

In immature thymocytes, Nir3, IP₃R2 and STIM2 are predominantly expressed. Upon weak TCR stimulation from self pMHC complex on thymic stromal cells, a small fraction of PIP₂ is cleaved by PLCγ1 which generates limited amounts of PA at the ER-PM junctions. Nir3 is recruited to the junctions and transfers PI to the PM which is important for PIP₂ replenishment. The whole Nir3-IP₃R2-STIM2 circuit is critical for the sustained TCR signaling upon weak TCR stimuli. In mature T cells. Nir2, IP₃R3 and STIM1 are major components of the SOCE pathway. The low sensitivity of the Nir2-IP₃R3-STIM1 contributes to the non-responsiveness of mature T cells to weak TCR stimuli. Upon strong TCR stimulation from foreign peptide-MHC ligands, a large amount of PIP₂ is metabolized which produces increased PA at the ER-PM junctions. Both Nir2 and Nir3 are recruited to the junctions in tandem. Nir2 transfers PI more efficiently than Nir3. Nir2 is the main PI transfer protein and Nir3 is less required for the PIP₂ replenishment in this context.