Phosphorylation of the carboxyl-terminal domain of the ζ1 subunit is not responsible for potentiation by TPA of the NMDA receptor channel

The carboxyl-terminal domain of the ζ1 subunit of the mouse NMDA receptor channel produced as a fusion protein with GST was phosphorylated in vitro by PKC. A mutant of the ζ1 subunit without serine or threonine residues in the carboxyl-terminal domain (ζ1-2-NST) was constructed and was expressed alone or together with the ε(lunate)2 subunit in Xenopus oocytes. Current responses of the ζ1-2-NST homomeric and ε(lunate)2/ζ1-2-NST heteromeric NMDA receptor channels were enhanced by treatment with TPA, a PKC activator, and the extents of potentiation were comparable with the corresponding wild-type channels. These results suggest that the phosphorylation of the carboxyl-terminal domain of the ζ1 subunit is not responsible for potentiation of NMDA receptor channels by the TPA treatment.