Differential functions of phospholipid binding and palmitoylation of tumour suppressor EWI2/PGRL.

The tumour suppressor EWI2 associates with tetraspanins and regulates tumour cell movement and proliferation. The short cytoplasmic domain of EWI2 is positively charged; five out of the ten residues of this domain are basic. In the present study we demonstrated that the EWI2 cytoplasmic tail interacts specifically with negatively charged PIPs (phosphatidylinositol phosphates), but not with other membrane lipids. The PIPs that interact with EWI2 cytoplasmic tail include PtdIns5P, PtdIns4P, PtdIns3P, PtdIns(3,5)P(2) and PtdIns(3,4)P(2). The binding affinity of PIPs to the EWI2 tail, however, is not solely based on charge because PtdIns5P, PtdIns4P and PtdIns3P have a higher affinity to EWI2 than PtdIns(3,5)P(2) and PtdIns(3,4)P(2) do. Mutation of either of two basic residue clusters in the EWI2 cytoplasmic tail abolishes PIP binding, and PIP binding is also determined by the position of basic residues in the EWI2 cytoplasmic tail. In addition, EWI2 is constitutively palmitoylated at the cytoplasmic cysteine residues located at the N-terminal of those basic residues. The PIP interaction is not required for, but appears to regulate, the palmitoylation, whereas palmitoylation is neither required for nor regulates the PIP interaction. Functionally, the PIP interaction regulates the stability of EWI2 proteins, whereas palmitoylation is needed for tetraspanin-EWI2 association and EWI2-dependent inhibition of cell migration and lamellipodia formation. For cell-cell adhesion and cell proliferation, the PIP interaction functions in opposition to the palmitoylation. In conclusion, the EWI2 cytoplasmic tail actively engages with the cell membrane via PIP binding and palmitoylation, which play differential roles in EWI2 functions.