TABLE 1.

Pharmacological specificity of photolabeling of residues in α1β3γ2 and α1β3 GABA_ARs by S-[³H]mTFD-MPPB, a convulsant, and R-[³H]mTFD-MPAB, an anesthetic (cpm/pmol of PTH-derivative)

The efficiency of photolabeling of a residue (in cpm/pmol) was calculated using Equation 4 (see under “Experimental Procedures”). N, the number of samples sequenced. The data are presented as the mean and individual values when two samples were sequenced or as mean (± S.D.) when three or four samples were sequenced. Other values were determined from the sequencing of single samples, with estimated uncertainties of <25%. The radiochemical specific activities of S-[³H]mTFD-MPPB and R-[³H]mTFD-MPAB are 50 and 38 Ci/mmol, respectively. ND means not determined.

Subunit–interface	Amino acid	S-[3H]mTFD-MPPB						R-[3H]mTFD-MPAB
		α1β3γ2 GABAAR			α1β3 GABAAR			α1β3γ2 GABAAR
		Control	GABA	Bicuculline	Control	GABA	Bicuculline	GABA	Bicuculline
γ+ M2	Ser-280	4000a	250a	ND	X	X	X	<50	<50
β+ M3	Met-286	36 ± 10 (n = 4)	56 ± 17 (n = 4)	45 ± 8 (n = 3)	80	100	70	100	90
β+ M3	Phe-289	210 ± 10 (n = 4)	400 ± 40 (n = 4)	154 ± 27 (n = 3)	480	610	440	80	60
β+ M2	Thr-262	ND	460	80	220	470	300	170	100
β− M1	Met-227	73 (86, 59)	87 (77, 97)	67 (59, 74)	100	80	60	3110	2100
β− M1	Leu-231	39 (45, 33)	57 (57, 56)	26 (25, 26)	120	150	70	<70	<50
α+ M3	Tyr-294	100	ND	90	210	220	220	100	70
α− M1	Met-236	70	ND	50	100	90	ND	<100	<50

^a For γ2Ser-280, the cpm/pmol was determined from the data of Fig. 6A. For the control condition, the sample was sequenced with OPA treatment in cycle 4 to allow mass determination. For the +GABA condition, the sample was sequenced without OPA, and the cpm/pmol was calculated from the ³H released in cycle 21 and the mass of the control sample.