Negatively regulates TCR (T-cell antigen receptor)-mediated signaling in T-cells and FCER1 (high affinity immunoglobulin epsilon receptor)-mediated signaling in mast cells. Promotes CSK activation and recruitment to lipid rafts, which results in LCK inhibition. Inhibits immunological synapse formation by preventing dynamic arrangement of lipid raft proteins. May be involved in cell adhesion signaling. Interacts with FYN. When phosphorylated, interacts with CSK. Interacts with SLC9A3R1/EBP50. In resting T-cells, part of a PAG1-SLC9A3R1-MSN complex which is disrupted upon TCR activation. Interacts with LYN on plasma membrane lipid rafts. Identified in a complex with LYN and STAT3. Present in lipid rafts. Ubiquitously expressed. Present in germinal center B-cells, plasma cells, T-cells, monocytes and platelets (at protein level). Palmitoylated. Phosphorylated by FYN on Tyr-317 in resting T-cells; which promotes interaction with CSK. Dephosphorylated by PTPRC/CD45 upon TCR activation; which leads to CSK dissociation. May also be dephosphorylated by PTPN11. Hyperphosphorylated in mast cells upon FCER1 activation. Phosphorylated by LYN.