Arévalo Sergio, Flores Enrique
Instituto de Bioquímica Vegetal y Fotosíntesis, CSIC and Universidad de Sevilla, Seville, Spain.
J Bacteriol. 2021 Jun 8;203(13):e0008121. doi: 10.1128/JB.00081-21.
Multicellular heterocyst-forming cyanobacteria, such as , grow as chains of cells forming filaments that, under diazotrophic conditions, contain two cell types: vegetative cells that perform oxygenic photosynthesis and N-fixing heterocysts. Along the filament, the intercellular septa contain a thick peptidoglycan layer that forms septal disks. Proteinaceous septal junctions connect the cells in the filament traversing the septal disks through nanopores. The operon encodes proteins needed to make long filaments in . FraC and FraD, located at the intercellular septa, are involved in the formation of septal junctions. Using a superfolder-green fluorescent protein (GFP) fusion, we found in this study that FraE is mainly localized to the poles of the heterocysts, consistent with the requirement of FraE for constriction of the heterocyst poles to form the "heterocyst neck." A insertional mutant was impaired by 22% to 38% in transfer of fluorescent calcein from vegetative cells to heterocysts. Septal disks were inspected in murein sacculi from heterocyst-enriched preparations. Unexpectedly, the diameter of the nanopores in heterocyst septa was about 1.5- to 2-fold larger than in vegetative cell septa. The number of these nanopores was 76% and 6% of the wild-type number in and mutants, respectively. Our results show that FraE is mainly involved in heterocyst maturation, whereas FraC and FraD are needed for the formation of the large nanopores of heterocyst septa, as they are for vegetative cell nanopores. Additionally, arrays of small pores conceivably involved in polysaccharide export were observed close to the septal disks in the heterocyst murein sacculus preparations. Intercellular communication, an essential attribute of multicellularity, is required for diazotrophic growth in heterocyst-forming cyanobacteria such as , in which the cells are connected by proteinaceous septal junctions that are structural analogs of metazoan connexons. The septal junctions allow molecular intercellular diffusion traversing the septal peptidoglycan through nanopores. In the operon encodes septal proteins involved in intercellular communication. FraC and FraD are components of the septal junctions along the filament, whereas here we show that FraE is mainly present at the heterocyst poles. We found that the intercellular septa in murein sacculi from heterocysts contain nanopores that are larger than those in vegetative cells, establishing a previously unknown difference between heterocyst and vegetative cell septa in .
多细胞异形胞形成蓝细菌，如 ，以细胞链形式生长形成丝状体，在固氮条件下，丝状体包含两种细胞类型：进行产氧光合作用的营养细胞和固氮异形胞。沿着丝状体，细胞间隔膜含有一层厚厚的肽聚糖层，形成隔膜盘。蛋白质性的隔膜连接通过纳米孔穿过隔膜盘连接丝状体中的细胞。 操纵子编码在 中形成长丝所需的蛋白质。位于细胞间隔膜处的FraC和FraD参与隔膜连接的形成。在本研究中，我们使用超折叠绿色荧光蛋白（GFP）融合蛋白发现，FraE主要定位于异形胞的两极，这与FraE对异形胞两极收缩以形成“异形胞颈部”的需求一致。一个 插入突变体在荧光钙黄绿素从营养细胞向异形胞的转移中受损22%至38%。在富含异形胞的制剂的胞壁质囊泡中检查隔膜盘。出乎意料的是，异形胞隔膜中纳米孔的直径比营养细胞隔膜中的大约大1.5至2倍。在 和 突变体中，这些纳米孔的数量分别是野生型数量的76%和6%。我们的结果表明，FraE主要参与异形胞成熟，而FraC和FraD是异形胞隔膜大纳米孔形成所必需的，就像它们对营养细胞纳米孔一样。此外，在异形胞胞壁质囊泡制剂中，在隔膜盘附近观察到了可能参与多糖输出的小孔阵列。细胞间通讯是多细胞性的一个基本属性，对于像 这样的异形胞形成蓝细菌的固氮生长是必需的，其中细胞通过蛋白质性的隔膜连接相连，这些隔膜连接是后生动物连接子的结构类似物。隔膜连接允许分子通过纳米孔穿过隔膜肽聚糖进行细胞间扩散。在 中， 操纵子编码参与细胞间通讯的隔膜蛋白。FraC和FraD是沿着丝状体的隔膜连接的组成部分，而在这里我们表明FraE主要存在于异形胞的两极。我们发现，来自异形胞的胞壁质囊泡中的细胞间隔膜含有比营养细胞中更大的纳米孔，这在 中建立了异形胞和营养细胞隔膜之间以前未知的差异。
