Sutton J M, Lea E J, Downie J A
John Innes Institute, John Innes Centre, Norwich, United Kingdom.
Proc Natl Acad Sci U S A. 1994 Oct 11;91(21):9990-4. doi: 10.1073/pnas.91.21.9990.
The secreted nodulation-signaling protein NodO was purified from the supernatant of cultures of Rhizobium leguminosarum biovar viciae. The native protein has a M(r) of approximately 67,000, suggesting that it exists as a dimer since the DNA sequence predicts a M(r) of 30,002. Pure NodO protein had no protease, pectinase, or cellulase activity, and no binding was observed to lipooligosaccharide nodulation factors. Although NodO is relatively hydrophilic, it appeared to insert into liposomes and was protected by liposomes from proteolytic cleavage. When added to planar lipid bilayers, NodO formed cation-selective channels that allowed the movement of monovalent cations (K+ and Na+) across the membrane. NodO is a Ca(2+)-binding protein; in the presence of high concentrations of Ca2+, channel activity was reduced. We hypothesize that NodO plays a role in nodulation signaling by stimulating uptake of nodulation factors or by forming cation-specific channels that function synergistically with the proposed lipooligosaccharide-induced depolarization of the plasma membrane of leguminous plants.
分泌型结瘤信号蛋白NodO是从豌豆根瘤菌蚕豆生物变种培养物的上清液中纯化得到的。天然蛋白的相对分子质量约为67,000，这表明它以二聚体形式存在，因为DNA序列预测其相对分子质量为30,002。纯NodO蛋白没有蛋白酶、果胶酶或纤维素酶活性，并且未观察到与脂寡糖结瘤因子的结合。尽管NodO相对亲水，但它似乎能插入脂质体，并受到脂质体的保护而免受蛋白水解切割。当添加到平面脂质双分子层中时，NodO形成阳离子选择性通道，允许单价阳离子（K +和Na +）跨膜移动。NodO是一种Ca(2 +)结合蛋白；在高浓度Ca2 +存在下，通道活性降低。我们推测，NodO通过刺激结瘤因子的摄取或通过形成与拟议的脂寡糖诱导的豆科植物质膜去极化协同作用的阳离子特异性通道，在结瘤信号传导中发挥作用。
