Flip and flop: a molecular determinant for AMPA receptor channel opening

Alternative splicing in the extracellular ligand binding domain of the AMPA receptors generates two variants, i.e., flip and flop. The flop variant of the GluR2 AMPA receptor is known to desensitize faster than the flip counterpart, whereas the GluR1 flip and flop variants exhibit the same rate of desensitization. However, whether the alternative splicing affects the channel opening kinetic properties of these receptors is unknown. Using a laser-pulse photolysis technique, we have characterized the channel opening kinetic mechanism for the flip and flop channels of GluR1 and GluR2, respectively. We find that the flop variant of GluR2 opens the channel, following the binding of glutamate, with the same rate as the flip channel, but closes its channel more rapidly. The difference in the kinetic properties between the two receptor isoforms can be described by a model we proposed previously in which the channel closing rate, a measure of the stability of the open channel state, controls an apparent tendency of the channel to desensitize, most likely, through the closed channel state. Specifically, the flop sequence of GluR2 promotes the channel to close more rapidly and consequently to desensitize with a faster rate than the flip sequence. For GluR1, the alternative splicing does not seem to affect the channel opening kinetics, since the flip and flop variants of GluR1 have the same channel opening rate, and the same channel closing rate. As expected and indeed observed, the flop variant desensitizes with the same rate as the flip variant does. On the basis of these results, we hypothesize that the flip/flop sequence cassette of AMPA receptors, in a sequence-dependent manner, regulates the rate of the channel closing process, in the microsecond time domain, through which it further regulates the channel desensitization in the millisecond time region.