Selão Tiago Toscano, Zhang Lifang, Ariöz Candan, Wieslander Åke, Norling Birgitta
School of Biological Sciences, Nanyang Technological University, Singapore, Singapore.
Department of Biochemistry and Biophysics, Arrhenius Laboratories, Stockholm University, Stockholm, Sweden.
PLoS One. 2014 Feb 6;9(2):e88153. doi: 10.1371/journal.pone.0088153. eCollection 2014.
Synthesis of monogalactosyldiacylglycerol (GalDAG) and digalactosyldiacylglycerol (GalGalDAG), the major membrane lipids in cyanobacteria, begins with production of the intermediate precursor monoglucosyldiacylglycerol (GlcDAG), by monoglucosyldiacylglycerol synthase (MGS). In Synechocystis sp. PCC6803 (Synechocystis) this activity is catalyzed by an integral membrane protein, Sll1377 or MgdA. In silico sequence analysis revealed that cyanobacterial homologues of MgdA are highly conserved and comprise a distinct group of lipid glycosyltransferases. Global regulation of lipid synthesis in Synechocystis and, more specifically, the influence of the lipid environment on MgdA activity have not yet been fully elucidated. Therefore, we purified membrane subfractions from this organism and assayed MGS activity in vitro, with and without different lipids and other potential effectors. Sulfoquinovosyldiacylglycerol (SQDAG) potently stimulates MgdA activity, in contrast to other enzymes of a similar nature, which are activated by phosphatidylglycerol instead. Moreover, the final products of galactolipid synthesis, GalDAG and GalGalDAG, inhibited this activity. Western blotting revealed the presence of MgdA both in plasma and thylakoid membranes, with a high specific level of the MgdA protein in the plasma membrane but highest MGS activity in the thylakoid membrane. This discrepancy in the subcellular localization of enzyme activity and protein may indicate the presence of either an unknown regulator and/or an as yet unidentified MGS-type enzyme. Furthermore, the stimulation of MgdA activity by SQDAG observed here provides a new insight into regulation of the biogenesis of both sulfolipids and galactolipids in cyanobacteria.
单半乳糖基二酰基甘油（GalDAG）和双半乳糖基二酰基甘油（GalGalDAG）是蓝藻中的主要膜脂，其合成始于单葡萄糖基二酰基甘油合酶（MGS）产生中间前体单葡萄糖基二酰基甘油（GlcDAG）。在集胞藻PCC6803（集胞藻）中，该活性由整合膜蛋白Sll1377或MgdA催化。计算机序列分析表明，MgdA的蓝藻同源物高度保守，构成了一组独特的脂质糖基转移酶。集胞藻中脂质合成的全局调控，更具体地说，脂质环境对MgdA活性的影响尚未完全阐明。因此，我们从该生物体中纯化了膜亚组分，并在有和没有不同脂质及其他潜在效应物的情况下体外测定了MGS活性。与其他性质相似但由磷脂酰甘油激活的酶相反，磺基喹喔啉基二酰基甘油（SQDAG）能有效刺激MgdA活性。此外，半乳糖脂合成的终产物GalDAG和GalGalDAG抑制了这种活性。蛋白质免疫印迹显示质膜和类囊体膜中均存在MgdA，质膜中MgdA蛋白的特异性水平较高，但类囊体膜中的MGS活性最高。酶活性和蛋白质在亚细胞定位上的这种差异可能表明存在未知的调节因子和/或尚未鉴定的MGS型酶。此外，此处观察到的SQDAG对MgdA活性的刺激为蓝藻中硫脂和半乳糖脂生物合成的调控提供了新的见解。
