Control of glutamate receptor 2 (GluR2) translational initiation by its alternative 3' untranslated regions

Four major glutamate receptor 2 (GluR2) transcripts differing in size (approximately 4 and approximately 6 kilobases) due to alternative 3' untranslated regions (UTRs), and also containing alternative 5'UTRs, exist in the brain. Both the long 5'UTR and long 3'UTR repress translation of GluR2 mRNA; repression by the 3'UTR is relieved after seizures. To understand the mechanism of translational repression, we used rabbit reticulocyte lysates as an in vitro translation system to examine the expression profiles of firefly reporter mRNAs bearing alternative combinations of GluR2 5'UTR and 3'UTR in the presence of inhibitors of either translational elongation or initiation. Translation of reporter mRNAs bearing the long GluR2 3'UTR was insensitive to low concentrations of the translation elongation inhibitors cycloheximide (0.7-70 nM) and anisomycin (7.5-750 nM), in contrast to a reporter bearing the short 3'UTR, which was inhibited. These data suggest that the rate-limiting step for translation of GluR2 mRNA bearing the long 3'UTR is not elongation. Regardless of the GluR2 UTR length, translation of all reporter mRNAs was equally sensitive to desmethyl-desamino-pateamine A (0.2-200 nM), an initiation inhibitor. Kasugamycin, which can facilitate recognition of certain mRNAs by ribosomes leading to alternative initiation, had no effect on translation of a capped reporter bearing both short 5'UTR and short 3'UTR, but increased the translation rate of reporters bearing either the long GluR2 5'UTR or long 3'UTR. Our findings suggest that both the long 5'UTR and long 3'UTR of GluR2 mRNA repress translation at the initiation step.