Bienert Ralf, Baier Kerstin, Volkmer Rudolf, Lockau Wolfgang, Heinemann Udo
Crystallography Group, Max-Delbrück Center for Molecular Medicine, D-13125 Berlin.
J Biol Chem. 2006 Feb 24;281(8):5216-23. doi: 10.1074/jbc.M507243200. Epub 2005 Dec 15.
Cyanobacterial light-harvesting complexes, the phycobilisomes, are proteolytically degraded when the organisms are starved for combined nitrogen, a process referred to as chlorosis or bleaching. Gene nblA, present in all phycobilisome-containing organisms, encodes a protein of about 7 kDa that plays a key role in phycobilisome degradation. The mode of action of NblA in this degradation process is poorly understood. Here we presented the 1.8-A crystal structure of NblA from Anabaena sp. PCC 7120. In the crystal, NblA is present as a four-helix bundle formed by dimers, the basic structural units. By using pull-down assays with immobilized NblA and peptide scanning, we showed that NblA specifically binds to the alpha-subunits of phycocyanin and phycoerythrocyanin, the main building blocks of the phycobilisome rod structure. By site-directed mutagenesis, we identified amino acid residues in NblA that are involved in phycobilisome binding. The results provided evidence that NblA is directly involved in phycobilisome degradation, and the results allowed us to present a model that gives insight into the interaction of this small protein with the phycobilisomes.
蓝藻光捕获复合体——藻胆体，在生物体缺乏化合态氮时会被蛋白酶降解，这个过程称为缺绿病或白化。存在于所有含藻胆体的生物体中的nblA基因，编码一种约7 kDa的蛋白质，该蛋白质在藻胆体降解中起关键作用。人们对NblA在这个降解过程中的作用方式了解甚少。在此，我们展示了来自鱼腥藻PCC 7120的NblA的1.8 Å晶体结构。在晶体中，NblA以由二聚体形成的四螺旋束形式存在，二聚体是基本结构单元。通过使用固定化NblA的下拉实验和肽扫描，我们表明NblA特异性结合藻胆体杆状结构的主要组成部分——藻蓝蛋白和藻红蛋白的α亚基。通过定点诱变，我们确定了NblA中参与藻胆体结合的氨基酸残基。这些结果提供了证据，证明NblA直接参与藻胆体降解，并且这些结果使我们能够提出一个模型，深入了解这种小蛋白质与藻胆体的相互作用。
