Alves Isabel D, Salamon Zdzislaw, Hruby Victor J, Tollin Gordon
Department of Chemistry, University of Arizona, Tucson, Arizona 85721, USA.
Biochemistry. 2005 Jun 28;44(25):9168-78. doi: 10.1021/bi050207a.
A growing body of evidence supports the idea that the plasma membrane bilayer is characterized by a laterally inhomogeneous mixture of lipids, having an organized structure in which lipid molecules segregate into small domains or patches. Such microdomains are characterized by high contents of sphingolipids that form thicker liquid-ordered regions that are resistant to extraction with nonionic detergents. The existence of lipid lateral segregation has been demonstrated in both model and biological membranes, although its role in protein sorting and membrane function still remains unclear. In these studies, plasmon-waveguide resonance (PWR) spectroscopy was employed to investigate the properties of microdomains in a model system consisting of a solid-supported lipid bilayer composed of a 1:1 mixture of palmitoyloleoylphosphatidylcholine (POPC) and brain sphingomyelin (SM), and their influence on the partitioning and functioning of the human delta opioid receptor (hDOR), a G-protein coupled receptor (GPCR). Resonance signals corresponding to two microdomains (POPC-rich and SM-rich) were observed in such bilayers, and the sorting of the receptor into each domain was highly dependent on the type of ligand that was bound. When no ligand was bound, the receptor was incorporated preferentially into the POPC-rich domain; when an agonist or antagonist was bound, the receptor was incorporated preferentially into the SM-rich component, although with a 2-fold greater propensity for this microdomain in the case of the agonist. Binding of G-protein to the agonist-bound receptor in the SM-rich domain occurred with a 30-fold higher affinity than binding to the receptor in the PC-rich domain. The binding of the agonist to an unliganded receptor in the bilayer produced receptor trafficking from the PC-rich to the SM-rich component. Since the SM-rich domain is thicker than the PC-rich domain, and previous studies with the hDOR have shown that the receptor is elongated upon agonist activation, we propose that hydrophobic matching between the receptor and the lipid is a driving force for receptor trafficking to the SM-rich component.
越来越多的证据支持这样一种观点，即质膜双层的特征是脂质的横向不均匀混合物，具有一种有组织的结构，其中脂质分子会分离成小区域或斑块。这种微区的特征是鞘脂含量高，形成较厚的液态有序区域，这些区域对非离子去污剂的提取具有抗性。脂质横向分离在模型膜和生物膜中均已得到证实，尽管其在蛋白质分选和膜功能中的作用仍不清楚。在这些研究中，表面等离子体波导共振（PWR）光谱被用于研究由棕榈酰油酰磷脂酰胆碱（POPC）和脑鞘磷脂（SM）按1:1混合物组成的固体支持脂质双层模型系统中微区的性质，以及它们对人δ阿片受体（hDOR，一种G蛋白偶联受体（GPCR））的分配和功能的影响。在这种双层膜中观察到了对应于两个微区（富含POPC和富含SM）的共振信号，并且受体在每个区域的分选高度依赖于所结合配体的类型。当没有配体结合时，受体优先整合到富含POPC的区域；当结合激动剂或拮抗剂时，受体优先整合到富含SM的组分中，尽管对于激动剂而言，这种微区的倾向是其两倍。G蛋白与富含SM区域中结合激动剂的受体的结合亲和力比与富含PC区域中受体的结合亲和力高30倍。激动剂与双层膜中未结合配体的受体的结合导致受体从富含PC的组分转运到富含SM的组分。由于富含SM的区域比富含PC的区域厚，并且先前对hDOR的研究表明受体在激动剂激活时会伸长，我们提出受体与脂质之间的疏水匹配是受体转运到富含SM组分的驱动力。
