Isolation of TM1 residues that modulate desensitization. A, The left column shows additional δ-γ2L chimeras used to determine the role of TM1 in desensitization. The indicated chimeras (γ2L sequence iswhite, δ subunit sequence is gray) were expressed with α1 and β3 subunits. Representative traces are shown in middle column. Bar graphs are as in Figure 2. Data from the rapidly desensitizing M1e chimera (from Fig. 2) is shown again for comparison. The asterisk indicates significant difference from both αβδ (p < 0.01) and αβγ (p < 0.05) receptors. Horizontal calibration: 400 msec; vertical calibration: 4.5 pA for M1i, 22 pA for M1p, 35 pA for M1pre-iso, and 68 pA for M1e.B, Amino acid sequence alignment of the first transmembrane domain of the δ and γ2L subunits. N-terminal end (extracellular side) is to the left. The δ subunit sequence is R229 through I255; the γ2L subunit sequence is R231 through I257. Identical residues shared by these subunits areshaded. The lines witharrows indicate chimera splice positions where δ subunit sequence ended and γ2L subunit sequence began, corresponding to the four chimeras shown in (A). Theasterisks indicate the only residues that differ between the M1e and M1pre-iso chimeras.