Department of Biochemistry & Molecular Medicine and Zilkha Neurogenetic Institute, Keck School of Medicine, University of Southern California , 1501 San Pablo Street, Los Angeles, California 90033, United States.
Department of Life Sciences, Korea University , 145 Anam-Ro, Seongbuk-Gu, Seoul 136-701, South Korea.
J Phys Chem B. 2018 Jan 25;122(3):1185-1194. doi: 10.1021/acs.jpcb.7b11227. Epub 2018 Jan 11.
The function of membrane proteins relies on a defined orientation of protein relative to lipid. In apparent correlation to protein anchoring, tryptophan residues are enriched in the lipid headgroup region. To characterize the thermodynamic and structural basis of this relationship in α-helical membrane proteins, we examined the role of three conserved tryptophans in the folding of the heterodimeric integrin αIIbβ3 transmembrane (TM) complex in phospholipid bicelles and mammalian membranes. In the homogenous lipid environment of bicelles, tryptophan was replaceable by residues of distinct polarities. The appropriate polarity was guided by the electrostatic potential of the tryptophan surrounding, suggesting that tryptophan can complement diverse environments by adjusting the orientation of its anisotropic side chain to achieve site-specific anchoring. As a sole membrane anchor, tryptophan made a contribution of 0.4 kcal/mol to TM complex stability in bicelles. In membranes, it proved more difficult to replace tryptophan even by tyrosine, indicating a superior capacity to interact with heterogeneous lipids of biological membranes. Interestingly, at intracellular TM helix ends, where integrin activation is initiated, sequence motifs that interact with lipids via opposing polarity patterns were found to restrict TM helix orientations beyond tryptophan anchoring. In contrast to bicelles, phenylalanine became the least accepted substitute in membranes, demonstrating an increased role of the hydrophobic effect. Altogether, our study implicates a wide amphiphilic range of tryptophan, membrane complexity, and the hydrophobic effect to be important factors in tryptophan membrane anchoring.
膜蛋白的功能依赖于蛋白质相对于脂质的特定取向。显然与蛋白质锚定相关，色氨酸残基在脂质头部区域富集。为了描述 α-螺旋膜蛋白中这种关系的热力学和结构基础，我们研究了三个保守色氨酸在整合素 αIIbβ3 跨膜（TM）复合物折叠中的作用在磷脂双体和哺乳动物膜中的异二聚体。在双体的均匀脂质环境中，色氨酸可以被不同极性的残基取代。适当的极性由色氨酸周围的静电势引导，这表明色氨酸可以通过调整其各向异性侧链的方向来适应不同的环境，从而实现特定位置的锚定。作为唯一的膜锚，色氨酸在双体中对 TM 复合物的稳定性贡献了 0.4kcal/mol。在膜中，即使是用酪氨酸取代色氨酸也更加困难，这表明其与生物膜异质脂质相互作用的能力更强。有趣的是，在细胞内 TM 螺旋末端，整合素的激活起始于此，与通过相反极性模式与脂质相互作用的序列基序限制了 TM 螺旋的取向，超越了色氨酸的锚定。与双体相比，在膜中苯丙氨酸成为最不被接受的取代物，这表明疏水性效应的作用增加。总的来说，我们的研究表明，广泛的色氨酸两亲性范围、膜复杂性和疏水性效应是色氨酸膜锚定的重要因素。
