Functional heterogeneity of ionotropic glutamate receptors arises not only from the existence of many subunits and isoforms, but also from combinatorial assembly creating channels with distinct properties. This heteromerization is subtype restricted and thought to be determined exclusively by the proximal extracellular N-terminal domain of the subunits. However, using functional assays for heteromer formation, we show that, besides the N-terminal domain, the membrane sector and the C-terminal part of S2 are critical determinants for the formation of functional channels. Our results are compatible with a model where the N-terminal domain only mediates the initial subunit associations into dimers, whereas for the assembly of the full functional tetramer, compatibility of the other regions is required.
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We thank S. Heinemann for his support at the early stages of the project and for the gifts of GluR3(Q621R) and F GluR2 R , R. Petroski and N. Brose for their help with preliminary experiments, M. Geylis for excellent technical assistance, and A. Kluger for help with statistics. We further express our gratitude to B. Kanner for his help in revising the manuscript and to J. Lerma, R. Rahamimoff, Z. Siegfried, M. Treinin, and R. Wenthold for their comments. This work was supported by THE ISRAEL SCIENCE FOUNDATION (grant no. 640/97) and by the Bernard Katz Minerva Center for Cell Biophysics.