Ryan S E, Demers C N, Chew J P, Baenziger J E
Department of Biochemistry, University of Ottawa, Ottawa, Ontario K1H 8M5, Canada.
J Biol Chem. 1996 Oct 4;271(40):24590-7. doi: 10.1074/jbc.271.40.24590.
The effects of both neutral and anionic lipids on the structure of the nicotinic acetylcholine receptor (nAChR) have been probed using infrared difference spectroscopy. The difference between infrared spectra of the nAChR recorded using the attenuated total reflectance technique in the presence and absence of the neurotransmitter analog, carbamylcholine, exhibits a complex pattern of positive and negative bands that provides a spectral map of the structural changes that occur in the nAChR upon ligand binding and subsequent desensitization. This spectral map is essentially identical in difference spectra recorded from native, native alkaline-extracted, and affinity-purified nAChR reconstituted into either soybean asolectin or egg phosphatidylcholine membranes containing both neutral and anionic lipids. This result suggests both a similar structure of the nAChR and a similar resting to desensitized conformational change in each membrane environment. In contrast, difference spectra recorded from the nAChR reconstituted into egg phosphatidylcholine membranes lacking neutral and/or anionic lipids all exhibit an essentially identical pattern of band intensity variations, which is similar to the pattern of variations observed in difference spectra recorded in the continuous presence of the desensitizing local anesthetic, dibucaine. The difference spectra suggest that the main effect of both neutral and anionic lipids in a reconstituted egg phosphatidylcholine membrane is to help stabilize the nAChR in a resting conformation. In the absence of neutral and/or anionic lipids, the nAChR is converted into an alternate conformation that appears to be analogous to the local anesthetic-induced desensitized state. Significantly, the proportion of receptors found in the resting versus the putative desensitized state appears to be dependent upon the final lipid composition of the reconstituted membrane. A lipid-dependent modulation of the equilibrium between a channel-active resting and channel-inactive desensitized state may account for the modulations of nAChR activity that are observed in different lipid membranes.
利用红外差示光谱法探究了中性和阴离子脂质对烟碱型乙酰胆碱受体（nAChR）结构的影响。在存在和不存在神经递质类似物氨甲酰胆碱的情况下，使用衰减全反射技术记录的nAChR红外光谱之间的差异，呈现出正负谱带的复杂模式，这提供了一幅nAChR在配体结合及随后脱敏过程中发生的结构变化的光谱图。从天然的、天然碱性提取的以及亲和纯化的nAChR重构到含有中性和阴离子脂质的大豆asolectin或鸡蛋磷脂酰胆碱膜中所记录的差示光谱中，该光谱图基本相同。这一结果表明在每种膜环境中nAChR具有相似的结构，以及从静息态到脱敏态的构象变化相似。相比之下，重构到缺乏中性和/或阴离子脂质的鸡蛋磷脂酰胆碱膜中的nAChR所记录的差示光谱，均呈现出基本相同的谱带强度变化模式，这与在持续存在脱敏局部麻醉剂丁卡因的情况下所记录的差示光谱中观察到的变化模式相似。差示光谱表明，在重构的鸡蛋磷脂酰胆碱膜中，中性和阴离子脂质的主要作用是帮助将nAChR稳定在静息构象。在缺乏中性和/或阴离子脂质时，nAChR会转变为一种似乎类似于局部麻醉剂诱导的脱敏状态的替代构象。值得注意的是，处于静息态与假定脱敏态的受体比例似乎取决于重构膜的最终脂质组成。通道活性静息态和通道非活性脱敏态之间平衡的脂质依赖性调节，可能解释了在不同脂质膜中观察到的nAChR活性调节现象。
