Department of Applied Sciences, University Parthenope of Naples, Centro Direzionale Isola C4, I-80143 Naples, Italy.
Proc Natl Acad Sci U S A. 2010 Mar 2;107(9):4135-40. doi: 10.1073/pnas.0910632107. Epub 2010 Feb 10.
Aquaporins are homotetrameric channel proteins, which allow the diffusion of water and small solutes across biological membranes. According to their transport function, aquaporins can be divided into "orthodox aquaporins", which allow the flux of water molecules only, and "aquaglyceroporins", which facilitate the diffusion of glycerol and other small solutes in addition to water. The contribution of individual residues in the pore to the selectivity of orthodox aquaporins and aquaglyceroporins is not yet fully understood. To gain insights into aquaporin selectivity, we focused on the sequence variation and electrostatics of their channels. The continuum Poisson-Boltzmann electrostatic potential along the channel was calculated and compared for ten three-dimensional-structures which are representatives of different aquaporin subfamilies, and a panel of functionally characterized mutants, for which high-accuracy three-dimensional-models could be derived. Interestingly, specific electrostatic profiles associated with the main selectivity to water or glycerol could be identified. In particular: (i) orthodox aquaporins showed a distinctive electrostatic potential maximum at the periplasmic side of the channel around the aromatic/Arg (ar/R) constriction site; (ii) aquaporin-0 (AQP0), a mammalian aquaporin with considerably low water permeability, had an additional deep minimum at the cytoplasmic side; (iii) aquaglyceroporins showed a rather flat potential all along the channel; and (iv) the bifunctional protozoan PfAQP had an unusual all negative profile. Evaluation of electrostatics of the mutants, along with a thorough sequence analysis of the aquaporin pore-lining residues, illuminated the contribution of specific residues to the electrostatics of the channels and possibly to their selectivity.
水通道蛋白是同源四聚体通道蛋白，允许水分子和小溶质通过生物膜扩散。根据其转运功能，水通道蛋白可分为“经典水通道蛋白”，只允许水分子通过，和“水甘油通道蛋白”，除了水之外还促进甘油和其他小溶质的扩散。通道中个别残基对经典水通道蛋白和水甘油通道蛋白选择性的贡献尚未完全了解。为了深入了解水通道蛋白的选择性，我们专注于通道的序列变异和静电特性。我们计算并比较了代表不同水通道蛋白亚家族的十个三维结构和一组功能表征突变体的通道沿程连续泊松-玻尔兹曼静电势，对于这些突变体，我们可以推导出高精度的三维模型。有趣的是，可以识别与主要对水或甘油选择性相关的特定静电分布。特别是：（i）经典水通道蛋白在通道的周质侧环绕芳香族/精氨酸（ar/R）限制部位显示出独特的静电势最大值；（ii）水通道蛋白-0（AQP0），一种哺乳动物水通道蛋白，其水通透性相当低，在细胞质侧还有一个额外的深最低点；（iii）水甘油通道蛋白沿通道显示出相当平坦的电势；（iv）双功能原生动物 PfAQP 具有异常的全负轮廓。对突变体静电特性的评估，以及对水通道蛋白孔道衬里残基的全面序列分析，阐明了特定残基对通道静电特性及其选择性的贡献。
