GluR3 flip and flop: Differences in channel opening kinetics

Ample evidence from earlier studies of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors, GluR3 included, suggests that alternative splicing not only enriches AMPA receptor diversity but also, more importantly, creates receptor variants that are functionally different. However, it is not known whether alternative splicing affects the receptor channel opening that occurs in the microsecond time domain. Using a laser-pulse photolysis technique combined with whole-cell recording, we characterized the channel opening rate process for two alternatively spliced variants of GluR3, i.e., GluR3(flip) and GluR3(flop). We show that the alternative splicing that generates flip and flop variants of GluR3 receptors regulates the channel opening process by controlling the rate of channel closing but not the rate of channel opening or the glutamate binding affinity. Specifically, the flop variant closes its channel almost 4-fold faster than the flip variant. We therefore propose that the function of the flip-flop sequence module in the channel opening process of AMPA receptors is to stabilize the open channel conformation, presumably by its pivotal structural location. Furthermore, a comparison of the flip isoform among all AMPA receptor subunits, based on the magnitude of the channel opening rate constant, suggests that GluR3 is kinetically more similar to GluR2 and GluR4 than to GluR1.