Dean R M, Rivers R L, Zeidel M L, Roberts D M
Department of Biochemistry, Cellular, and Molecular Biology, Center for Legume Research, University of Tennessee, Knoxville 37996, USA.
Biochemistry. 1999 Jan 5;38(1):347-53. doi: 10.1021/bi982110c.
Infection of soybean roots by nitrogen-fixing Bradyrhizobium japonicum leads to expression of plant nodule-specific genes known as nodulins. Nodulin 26, a member of the major intrinsic protein/aquaporin (AQP) channel family, is a major component of the soybean symbiosome membrane (SM) that encloses the rhizobium bacteroid. To investigate the water and solute transport characteristics of nodulin 26, we purified the protein from SMs and reconstituted it into carboxyfluorescein-loaded liposomes for transport studies using stopped-flow spectrofluorimetry. Liposomes containing nodulin 26 exhibited a high osmotic permeability (Pf = 0. 012 +/- 0.0013 cm/s), a value fivefold higher than that obtained with control liposomes. Water flux through nodulin 26 showed a low activation energy (Ea) (4.07 kcal/mol) and was reduced 70% upon addition of 1 mM HgCl2. Reconstituted nodulin 26 exhibited a single-channel conductance of 3.8 +/- 2.5 x 10(-)15 cm3/s (n = 3), a value that is lower than other characterized AQPs. Nodulin 26 proteoliposomes also facilitate glycerol transport, showing a 43-fold higher rate of glycerol flux than control liposomes. This observation was supported by expression experiments in Xenopus oocytes that showed that nodulin 26 facilitated glycerol flux in a manner indistinguishable from the Escherichia coli GlpF glycerol facilitator. Consistent with the results of water transport, glycerol transport was inhibited by HgCl2 and showed a low Ea (4.43 kcal/mol). These results indicate that nodulin 26 is a multifunctional AQP that confers water and glycerol transport to the SM, and likely plays a role in osmoregulation during legume/rhizobia symbioses.
固氮慢生根瘤菌对大豆根的感染会导致植物根瘤特异性基因（即根瘤素）的表达。根瘤素26是主要内在蛋白/水通道蛋白（AQP）通道家族的成员，是包围根瘤菌类菌体的大豆共生体膜（SM）的主要成分。为了研究根瘤素26的水和溶质转运特性，我们从共生体膜中纯化了该蛋白，并将其重组成装载有羧基荧光素的脂质体，以便使用停流荧光光谱法进行转运研究。含有根瘤素26的脂质体表现出高渗透渗透率（Pf = 0.012 +/- 0.0013 cm/s），该值比对照脂质体高五倍。通过根瘤素26的水通量显示出低活化能（Ea）（4.07千卡/摩尔），并且在添加1 mM HgCl2后降低了70%。重组的根瘤素26表现出单通道电导为3.8 +/- 2.5 x 10(-)15 cm3/s（n = 3），该值低于其他已表征的水通道蛋白。根瘤素26蛋白脂质体也促进甘油转运，显示出甘油通量速率比对照脂质体高43倍。非洲爪蟾卵母细胞中的表达实验支持了这一观察结果，该实验表明根瘤素26促进甘油通量的方式与大肠杆菌GlpF甘油转运体无法区分。与水转运结果一致，甘油转运受到HgCl2的抑制，并且显示出低Ea（4.43千卡/摩尔）。这些结果表明，根瘤素26是一种多功能水通道蛋白，赋予共生体膜水和甘油转运能力，并且可能在豆科植物/根瘤菌共生过程中的渗透调节中发挥作用。
