Aix-Marseille Université, CNRS, Laboratoire de Chimie Bactérienne UMR 7283, 31 chemin Joseph Aiguier, 13009 Marseille cedex 20, France.
Res Microbiol. 2013 Sep;164(7):710-7. doi: 10.1016/j.resmic.2013.05.004. Epub 2013 Jun 6.
In the filamentous heterocyst-forming cyanobacterium Anabaena PCC 7120, vegetative cells and heterocysts are interdependent on each other and engaged in exchanges of metabolites for survival when grown under diazotrophic conditions. In this organism, the periplasm appears to be continuous along each filament, with a shared outer membrane; however, barriers exist preventing free diffusion of the fluorescent protein GFP (27 kDa) targeted into the periplasmic space. Here we expressed a smaller fluorescent protein iLOV (≈ 13 kDa) fused to the All3333 (a putative homologue of NrtA) signal sequence corresponding to those recognized by the TAT protein translocation system, which exports iLOV to the periplasm of either heterocysts or vegetative cells. Fluorescence microscopy and immunoblot analysis indicated that the iLOV protein is translocated into the periplasm of the producing cell and properly processed, but does not diffuse to neighboring cells via the periplasm. Thus, periplasmic barriers appear to block diffusion of molecules with a size of 13 kDa, the minimum size tested thus far. Assuming that the physical barrier is the peptidoglycan sacculus, its pores might allow diffusion of molecules within the size range between the PatS pentapeptide and iLOV, thus between 0.53 kDa and 13 kDa.
在丝状异形胞形成蓝藻鱼腥藻 PCC 7120 中，营养细胞和异形胞相互依存，并在固氮条件下生长时进行代谢物交换以维持生存。在这个生物体中，周质似乎沿着每条纤维连续存在，具有共享的外膜；然而，存在阻止荧光蛋白 GFP（27 kDa）靶向周质空间的自由扩散的屏障。在这里，我们表达了一个较小的荧光蛋白 iLOV（≈13 kDa），与 All3333（一种假定的 NrtA 同源物）信号序列融合，该信号序列对应于 TAT 蛋白转运系统识别的信号序列，该系统将 iLOV 输出到异形胞或营养细胞的周质中。荧光显微镜和免疫印迹分析表明，iLOV 蛋白被转运到产生细胞的周质中并正确加工，但不会通过周质扩散到邻近细胞。因此，周质屏障似乎阻止了大小为 13 kDa 的分子的扩散，这是迄今为止测试的最小尺寸。假设物理屏障是肽聚糖囊泡，其孔可能允许 PatS 五肽和 iLOV 之间大小范围内的分子扩散，因此在 0.53 kDa 和 13 kDa 之间。
