Regulation of microtubule stability is crucial for the maintenance of cell structure and function. While the acetylation of α-tubulin lysine 40 by acetylase has been implicated in the regulation of microtubule stability, the in vivo functions of N-terminal acetyltransferases (NATs) involved in the acetylation of N-terminal amino acids are not well known. Here, we identify an N-terminal acetyltransferase, Mnat9, that regulates cell signaling and microtubule stability in Drosophila. Loss of Mnat9 causes severe developmental defects in multiple tissues. In the wing imaginal disc, Mnat9 RNAi leads to the ectopic activation of c-Jun N-terminal kinase (JNK) signaling and apoptotic cell death. These defects are suppressed by reducing the level of JNK signaling. Overexpression of Mnat9 can also inhibit JNK signaling. Mnat9 colocalizes with mitotic spindles, and its loss results in various spindle defects during mitosis in the syncytial embryo. Furthermore, overexpression of Mnat9 enhances microtubule stability. Mnat9 is physically associated with microtubules and shows a catalytic activity in acetylating N-terminal peptides of α- and β-tubulin in vitro. Cell death and tissue loss in Mnat9-depleted wing discs are restored by reducing the severing protein Spastin, suggesting that Mnat9 protects microtubules from its severing activity. Remarkably, Mnat9 mutated in the acetyl-CoA binding site is as functional as its wild-type form. We also find that human NAT9 can rescue Mnat9 RNAi phenotypes in flies, indicating their functional conservation. Taken together, we propose that Mnat9 is required for microtubule stability and regulation of JNK signaling to promote cell survival in developing Drosophila organs.

微管稳定性的调节对于维持细胞结构和功能至关重要。虽然乙酰化酶对 α-微管蛋白赖氨酸 40 的乙酰化与微管稳定性的调节有关，但参与 N 端氨基酸乙酰化的 N 端乙酰转移酶 (NAT) 的体内功能尚不清楚。在这里，我们确定了一种 N 末端乙酰转移酶 Mnat9，它可以调节果蝇中的细胞信号传导和微管稳定性。Mnat9 的缺失会导致多个组织出现严重的发育缺陷。在机翼成像盘中，Mnat9 RNAi导致c-Jun N-末端激酶（JNK）信号传导和凋亡细胞死亡的异位激活。这些缺陷是通过降低 JNK 信号水平来抑制的。Mnat9 的过表达也可以抑制 JNK 信号。Mnat9 与有丝分裂纺锤体共定位，其缺失导致合胞体胚胎有丝分裂过程中出现各种纺锤体缺陷。此外，Mnat9 的过表达增强了微管的稳定性。Mnat9 与微管物理结合，并在体外乙酰化 α-和 β-微管蛋白的 N 末端肽中显示出催化活性。通过减少切断蛋白 Spastin 可以恢复 Mnat9 耗尽的翼盘中的细胞死亡和组织损失，这表明 Mnat9 保护微管免受其切断活性。值得注意的是，在乙酰辅酶A 结合位点突变的 Mnat9 与其野生型形式一样具有功能。果蝇中的Mnat9 RNAi表型，表明它们的功能保守。总之，我们提出 Mnat9 是微管稳定性和调节 JNK 信号以促进发育中的果蝇器官细胞存活所必需的。