pH对磷脂膜中Slc11a1跨膜区二级结构和取向的影响

Effects of pH on Secondary Structure and Orientation of the Transmembrane Domains of Slc11a1 in Lipid Membrane

采用圆二色光谱、荧光光谱、红外衰减全反射光谱和差示扫描量热分析等方法对不同pH条件下膜蛋白Slc11a1(溶质转运蛋白家族11成员1)的第二、第三和第四跨膜区(TMD2～TMD4)在磷脂膜[二肉豆蔻酰磷脂酰胆碱(DMPC)和二肉豆蔻酰磷脂酰甘油(DMPG)的摩尔比为2∶1]中的二级结构和取向进行了研究.结果表明,TMD3的二级结构及在磷脂膜内的位置与pH密切相关,在pH=7时TMD3主要为β股结构,在膜中埋入较浅;而在pH=5.5时TMD3形成部分α螺旋结构,并较深地埋入膜中.对TMD3进行E139A突变后的结果证明,TMD3的这些性质与位于中间的谷氨酸的质子化性质密切相关.实验结果还表明,TMD2和TMD4在不同pH条件下都形成α螺旋结构并分别以26°和35°的倾斜角插入磷脂膜内,它们在磷脂膜内的位置基本不受pH影响.

Transmembrane protein Slc11a1（Solute carrier family 11 member 1） is a divalent metal ion transporter which includes 12 transmembrane domains（TMDs）.In this paper,the secondary structure and orientation of the peptides corresponding to TMD2,TMD3 and TMD4 in the phospholipid vesicles[n（DMPC）∶n（DMPG）=2∶ 1] at different pH values were investigated by circular dichroism,fluorescence,polarized attenuated total reflection Fourier transform infrared spectroscopies and differential scanning calorimetry.It was found that the secondary structure and positioning of TMD3 in lipid membrane are dependent on solution pH.TMD3 folds mainly as a β strand and is embedded in lipid membrane less deeply at pH=7,while it forms part of α-helix and inserts more deeply in lipid membrane at pH=5.5.These were attributed to the acidic residue Glu139 of TMD3,as indicated by the results of the E139A mutation of TMD3.Both TMD2 and TMD4 form predominantly α-helix structure at different pH values and insert into lipid membrane by tilt angles of 26° and 35°,respectively,and the positioning of them in membrane is basically independent of pH.