CD9-association with PIP(2) areas is regulated by a CD9 salt bridge.

Tetraspanins are membrane proteins involved in multiple cellular functions, which they regulate by means of tetraspanin-enriched microdomains. While many tetraspanin-associated processes are regulated by intracellular signaling, possibly through the crosstalk between intracellular tetraspanin segments and second messengers like PIP(2), this molecular crosstalk has remained largely unknown. To improve our understanding of this crosstalk, we investigate the possible relationship between an intracellular salt bridge of the tetraspanin CD9 and PIP(2). We find that CD9 readily associates with PIP(2)-rich areas, in contrast to its interaction partner EWI-2. The opening of the CD9 salt bridge lowers the abundance of CD9 in these PIP(2) areas. Instead, open-CD9 can be located in regions that are more strongly populated with EWI-2, promoting CD9-EWI-2 association. This study uncovers the process of an intracellular salt bridge regulating the association of CD9 with PIP(2)-enriched areas. This points toward a possible link between intracellular tetraspanin segments and signaling.