Fig. 4.

Predicted and aberrant arrangements of subunits included in tandem constructs. A, The theoretical arrangements of expressed single subunits and tandems and their ability to generate functional currents. Diagrams are as used in all figures;open areas represent NR1-derived elements; filled areas represent NR2A-derived elements. In all cases thesubunit missing a 90° segment is the start (the N terminal or head of a tandem), and the full circle is the end of the tandem. Arrangements shown in A are the organizations expected to be permitted by tandems without stretching or distorting the linker. B, Some of the possible ways that subunits in tandems may be able to violate the arrangements shown inA. The letters N and C are used to show the N and C terminals of the tandems forming a channel. Diagrams are as in A, except that gray-scale subunits indicate that the normally intracellular region of the tandem has flipped to the outside of the cell (flip in membrane). The side of the tandem used to form the pore has been chosen arbitrarily.C, Subunits within a single tandem may participate in the formation of two different channels. A chain of channels may be formed by subunits within a tandem participating in separate channels. The arrangement shown would allow a dimer of dimers organization of subunits within channels, although only the 2t→1 tandem is used.