Characterization of two fish glutamate receptor cDNA molecules: absence of RNA editing at the Q/R site.

Two cDNA clones encoding putative ionotropic glutamate receptor subunits were isolated from a brain cDNA library of a freshwater fish, Oreochromis sp. The deduced amino acid sequences of these two cDNAs, fGluR2 alpha and fGluR2 beta, display the highest sequence identity (85%) to that of the rat GluR2 (AMPA receptor subunit) and they contain an arginine codon at the Q/R editing site of the TM2 segment. Genomic sequence analysis of the exons encoding the TM2 reveals the presence of an arginine codon at the Q/R site, suggesting that the RNA editing mechanism acting in the mammalian GluR2 does not operate at the homologous site in these two fish genes. In contrast to the absence of RNA editing at the Q/R site, transcripts of fGluR2 alpha and fGluR2 beta are subjected to RNA editing at a second site, the R/G site. A splicing variant of fGluR2 alpha, fGluR2 alpha-c, with a shorter C-terminal sequence was found; however, no C-terminal splicing variant of fGluR2 beta was detected in the mature fish. Similar to the mammalian AMPA receptor, variants created by the alternate choice of flip and flop modules were found among transcripts of fGluR2 alpha-c and fGluR2 beta. The amino acid sequences of flip and flop modules of fGluR2 beta are identical to that of the rat GluR2, whereas the amino acid sequences of the flip and flop modules of fGluR2 alpha-c differ from the invariant consensus sequences of the rat AMPA receptor subunits.