Splice variants of the N-methyl-D-aspartate receptor NR1 identify domains involved in regulation by polyamines and protein kinase C

The N-methyl-D-aspartate (NMDA) receptor NR1 gene encodes RNA that is alternatively spliced to generate at least seven variants. The variants arise from splicing in or out of three exons; one encodes a 21-amino acid insert in the N-terminal domain, and two encode adjacent sequences of 37 and 38 amino acids in the C-terminal domain. Splicing out of the second C-terminal exon deletes a stop codon and results in an additional open reading frame encoding an unrelated sequence of 22 amino acids before arriving at a second stop codon. We denote the NR1 variants by the presence or absence of the three alternatively spliced exons (from 5′ to 3′); thus, NR1₁₁₁ has all three exons, NR1₀₀₀ has none, and NR1₁₀₀ has only the N-terminal exon. We report here electrophysiological characterization of six splice variants of the NR1 receptor expressed in Xenopus oocytes. NR1 receptors that lacked the N-terminal exon (NR1₀₀₀, NR1₀₁₀, and NR1₀₁₁) exhibited a relatively high affinity for NMDA (EC₅₀ ≈ 13 μM) and marked potentiation by spermine. In contrast, those receptor variants with the N-terminal insert (NR1₁₀₀, NR1₁₀₁, and NR1₁₁₁) showed a lower agonist affinity and little or no spermine potentiation at saturating glycine. All six variants showed spermine potentiation at low glycine and inhibition by spermine at more negative potentials. Variants differing only in the C-terminal domain differed little in agonist affinity and spermine potentiation. These findings indicate that the N-terminal insert either participates in agonist and polyamine binding domains or indirectly modifies their conformations. The splice variants differed in the extent to which they could be potentiated by activators of protein kinase C (PKC) from 3- to 20-fold. Presence of the N-terminal insert and absence of the C-terminal sequences increased potentiation by PKC. These findings identify the contributions of the separate polypeptide domains to modulation by polyamines and PKC and provide further support for the concept that subunit composition determines functional properties of NMDA receptors.