1. To identify amino acid residues that are important for spermine binding, we used site-directed mutagenesis to alter amino acids in and around a region of the NR1 subunit of the NMDA receptor that shows homology with pot D,a Polyamine binding protein from Escherichia coli. Mutated subunits, expressed in heteromeric and homomeric NMDA receptors, were studied by voltage-clamp recording in Xenopus oocytes. Mutation of two acidic residues (E339 or E342) to neutral amino acids reduced or abolished stimulation by spermine without affecting voltage-dependent block by spermine. Mutation of these residues also had modest effects on sensitivity to protons and to ifenprodil but did not alter sensitivity to glutamate and glycine or to voltage-dependent block by Mg^<2+>. Residue E342 in NR1 appears to be critical for spermine stimulation. Next, sixteen glutamate and aspartate residues, located in the first two thirds of the putative extracellular loop of the NR1A subunit, were individually mutate
d. This region of NR1A shows homology with bacterial amino acid binding proteins, a bacterial polyamine binding protein, and a bacterial spermidine acetyltransferase. Mutation of D669 to asparagine (D669N), alanine(D669A), orglutamate (D669E) abolished spermine stimulation. These mutations also markedly reduced inhibition by ifenprodil and by protons at NR1A/NR2B receptors. Mutations at NR1A (D669) had little or no effect on the potencies of glutamate and glycine and did not alter voltage-dependent block by Mg^<2+> or the "glycine-dependent" form of spermine stimulation. Surprisingly, the D669N and D669A mutations, but not the D669E mutation, reduced voltage-dependent block by spermine. D669 in NR1A could form part of a binding site for polyamines and ifenprodil and/or part of the proton sensor of the NMDA receptor.
2. The effects of several N-sulfonyl-polyamines, including N^1-dansyl-spermine (N^1-DanSpm) and N^1-(n-octanesulfonyl)-spermine (N^1-OsSpm), were studied at recombinant NMDA receptors expressed in Xenopus oocytes. N^1-DanSpm and N^1-OsSpm inhibited NMDA receptors and were about 1,000-fold more potent than spermine in oocytes voltage-clamped at -70 mV.Block by N^1-DanSpm and N^1-OsSpm was strongly voltage-dependent, being more pronounced at hyperpolarized membrane potentials. Less