Department of Biotechnology, The University of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo 113-8657, Japan.
Mol Microbiol. 2011 Jul;81(1):40-55. doi: 10.1111/j.1365-2958.2011.07700.x. Epub 2011 May 25.
Although exocytosis in fungal cells takes place at hyphal tips, there also seems a line of circumstantial evidence suggesting the occurrence of exocytosis at other sites of cells, such as septa. To investigate whether exocytosis takes place at fungal septa, we monitored dynamics of EGFP-fused α-amylase (AmyB-EGFP), the representative secretory enzyme of the filamentous fungus Aspergillus oryzae. We found that AmyB-EGFP accumulates in Spitzenkörper at hyphal tips as well as septal periplasm between the plasma membrane and cell walls. The septal accumulation of AmyB-EGFP was a rapid process, and required microtubules but not F-actin. Thus, this process is independent of exocytosis at hyphal tips that requires both microtubules and F-actin. In addition, fluorescence recovery after photobleaching (FRAP) analysis of EGFP-fused AoSnc1 revealed that secretory vesicles constitutively fuse with the septal plasma membrane. These results demonstrated that exocytosis takes place at septa in addition to hyphal tips. Analysis of two plasma membrane transporters, AoUapC and AoGap1, revealed that they preferentially accumulate at septa and the lateral plasma membrane with no clear accumulation at apical Spitzenkörper, suggesting that non-tip directed exocytosis is important for delivery of these proteins.
尽管真菌细胞中的胞吐作用发生在菌丝尖端，但也有一些间接证据表明胞吐作用也可能发生在细胞的其他部位，如隔膜。为了研究胞吐作用是否发生在真菌隔膜上，我们监测了融合 EGFP 的α-淀粉酶（AmyB-EGFP）的动力学，AmyB-EGFP 是丝状真菌米曲霉的代表性分泌酶。我们发现，AmyB-EGFP 在菌丝尖端的 Spitzenkörper 以及细胞膜和细胞壁之间的隔膜周质中积累。AmyB-EGFP 在隔膜上的积累是一个快速的过程，需要微管而不是 F-肌动蛋白。因此，这个过程与需要微管和 F-肌动蛋白的菌丝尖端的胞吐作用无关。此外，对融合 EGFP 的 AoSnc1 的荧光恢复后漂白（FRAP）分析表明，分泌囊泡与隔膜质膜持续融合。这些结果表明，除了菌丝尖端之外，胞吐作用也发生在隔膜上。对两种质膜转运蛋白 AoUapC 和 AoGap1 的分析表明，它们优先积累在隔膜和侧质膜上，而在顶端的 Spitzenkörper 上没有明显的积累，这表明非尖端定向的胞吐作用对于这些蛋白的运输很重要。
