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使用脂质双分子层方法对膜蛋白进行高通量结晶。

High-throughput crystallization of membrane proteins using the lipidic bicelle method.

作者信息

Ujwal Rachna, Abramson Jeff

机构信息

UCLA-DOE Institute for Genomics and Proteomics, University of California Los Angeles, USA.

出版信息

J Vis Exp. 2012 Jan 9(59):e3383. doi: 10.3791/3383.

Abstract

Membrane proteins (MPs) play a critical role in many physiological processes such as pumping specific molecules across the otherwise impermeable membrane bilayer that surrounds all cells and organelles. Alterations in the function of MPs result in many human diseases and disorders; thus, an intricate understanding of their structures remains a critical objective for biological research. However, structure determination of MPs remains a significant challenge often stemming from their hydrophobicity. MPs have substantial hydrophobic regions embedded within the bilayer. Detergents are frequently used to solubilize these proteins from the bilayer generating a protein-detergent micelle that can then be manipulated in a similar manner as soluble proteins. Traditionally, crystallization trials proceed using a protein-detergent mixture, but they often resist crystallization or produce crystals of poor quality. These problems arise due to the detergent's inability to adequately mimic the bilayer resulting in poor stability and heterogeneity. In addition, the detergent shields the hydrophobic surface of the MP reducing the surface area available for crystal contacts. To circumvent these drawbacks MPs can be crystallized in lipidic media, which more closely simulates their endogenous environment, and has recently become a de novo technique for MP crystallization. Lipidic cubic phase (LCP) is a three-dimensional lipid bilayer penetrated by an interconnected system of aqueous channels. Although monoolein is the lipid of choice, related lipids such as monopalmitolein and monovaccenin have also been used to make LCP. MPs are incorporated into the LCP where they diffuse in three dimensions and feed crystal nuclei. A great advantage of the LCP is that the protein remains in a more native environment, but the method has a number of technical disadvantages including high viscosity (requiring specialized apparatuses) and difficulties in crystal visualization and manipulation. Because of these technical difficulties, we utilized another lipidic medium for crystallization-bicelles (Figure 1). Bicelles are lipid/amphiphile mixtures formed by blending a phosphatidylcholine lipid (DMPC) with an amphiphile (CHAPSO) or a short-chain lipid (DHPC). Within each bicelle disc, the lipid molecules generate a bilayer while the amphiphile molecules line the apolar edges providing beneficial properties of both bilayers and detergents. Importantly, below their transition temperature, protein-bicelle mixtures have a reduced viscosity and are manipulated in a similar manner as detergent-solubilized MPs, making bicelles compatible with crystallization robots. Bicelles have been successfully used to crystallize several membrane proteins (Table 1). This growing collection of proteins demonstrates the versatility of bicelles for crystallizing both alpha helical and beta sheet MPs from prokaryotic and eukaryotic sources. Because of these successes and the simplicity of high-throughput implementation, bicelles should be part of every membrane protein crystallographer's arsenal. In this video, we describe the bicelle methodology and provide a step-by-step protocol for setting up high-throughput crystallization trials of purified MPs using standard robotics.

摘要

膜蛋白(MPs)在许多生理过程中发挥着关键作用,例如将特定分子泵过包围所有细胞和细胞器的原本不可渗透的膜双层。膜蛋白功能的改变会导致许多人类疾病和功能紊乱;因此,深入了解其结构仍然是生物学研究的一个关键目标。然而,膜蛋白的结构测定仍然是一个重大挑战,这通常源于它们的疏水性。膜蛋白有大量嵌入双层中的疏水区域。去污剂经常用于从双层中溶解这些蛋白质,形成蛋白质 - 去污剂胶束,然后可以以与可溶性蛋白质类似的方式进行处理。传统上,结晶试验使用蛋白质 - 去污剂混合物进行,但它们往往难以结晶或产生质量较差的晶体。这些问题的出现是由于去污剂无法充分模拟双层,导致稳定性差和异质性。此外,去污剂会屏蔽膜蛋白的疏水表面,减少可用于晶体接触的表面积。为了克服这些缺点,膜蛋白可以在脂质介质中结晶,脂质介质更接近模拟它们的内源性环境,并且最近已成为膜蛋白结晶的一种从头技术。脂质立方相(LCP)是一种由相互连接的水通道系统穿透的三维脂质双层。虽然单油酸甘油酯是首选脂质,但相关脂质如单棕榈油酸甘油酯和单vaccenin也已用于制备LCP。膜蛋白被掺入LCP中,在那里它们在三维空间中扩散并为晶核提供物质。LCP的一个很大优点是蛋白质保留在更天然的环境中,但该方法有许多技术缺点,包括高粘度(需要专门的设备)以及晶体可视化和操作方面的困难。由于这些技术难题,我们使用了另一种脂质介质进行结晶——双分子层微囊(图1)。双分子层微囊是通过将磷脂酰胆碱脂质(DMPC)与两亲分子(CHAPSO)或短链脂质(DHPC)混合形成的脂质/两亲分子混合物。在每个双分子层微囊盘中,脂质分子形成双层,而两亲分子排列在非极性边缘,兼具双层和去污剂的有益特性。重要的是,在其转变温度以下,蛋白质 - 双分子层微囊混合物的粘度降低,并且可以以与去污剂溶解的膜蛋白类似的方式进行操作,使得双分子层微囊与结晶机器人兼容。双分子层微囊已成功用于使几种膜蛋白结晶(表1)。越来越多的此类蛋白质证明了双分子层微囊在结晶来自原核和真核来源的α螺旋和β折叠膜蛋白方面的多功能性。由于这些成功以及高通量实施的简便性,双分子层微囊应该成为每个膜蛋白晶体学家的工具库的一部分。在本视频中,我们描述了双分子层微囊方法,并提供了使用标准机器人设置纯化膜蛋白高通量结晶试验的分步方案。

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