Alternative splicing generates distinct forms of the NMDA receptor subunit NR1. NR1 subunits with an N-terminal insert (termed N 1) form receptors in Xenopus oocytes with greatly reduced potentiation by spermine and Zn2+. Oocytes expressing NR1 receptors with N1 exhibited larger NMDA currents than oocytes expressing corresponding receptors without N 1. In the present study, we used mutational analysis to investigate structural features of the N 1 insert that control current amplitude and spermineandZn2+ potentiation. Neutralization of positive charges in N1 rescued spermine and Zn2+ potentiation. Positive charges in N 1 did not affect spermine or Zn2+ affinity. Neutralization of positive charges in N 1 diminished the responses to the level of NR 1 receptors lacking N 1. The positively charged N 1 may increase NMDA currents by causing a conformational change similar to that produced by spermine and Zn2+ in NR1 receptors lacking N1.
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