The mycobacterial cell envelope has a diderm structure, composed of an outer mycomembrane, an arabinogalactan-peptidoglycan cell wall, a periplasm, and an inner membrane. Lipomannan (LM) and lipoarabinomannan (LAM) are structural and immunomodulatory components of this cell envelope. LM/LAM biosynthesis involves a number of mannosyltransferases and acyltransferases, and MptA is an α1,6-mannosyltransferase involved in the final extension of the mannan chain. Recently, we reported the periplasmic protein LmeA being involved in the maturation of the mannan backbone in Mycobacterium smegmatis. Here, we examined the role of LmeA under stress conditions. We found that lmeA transcription was upregulated under two stress conditions: stationary growth phase and nutrient starvation. Under both conditions, LAM was decreased, but LM was relatively stable, suggesting that maintaining the cellular level of LM under stress is important. Surprisingly, the protein levels of MptA were decreased in an lmeA deletion (ΔlmeA) mutant under both stress conditions. The transcript levels of mptA in the ΔlmeA mutant were similar to or even higher than those in the wild type, indicating that the decrease of MptA protein was a posttranscriptional event. The ΔlmeA mutant was unable to maintain the cellular level of LM under stress, consistent with the decrease in MptA. Even during active growth, overexpression of LmeA led the cells to produce more LM and become more resistant to several antibiotics. Altogether, our study reveals the roles of LmeA in the homeostasis of the MptA mannosyltransferase, particularly under stress conditions, ensuring the stable expression of LM and the maintenance of cell envelope integrity.

中文翻译：

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LmeA，一种分枝杆菌周质蛋白，在压力下维持甘露糖基转移酶 MptA 及其产物脂甘露聚糖的作用

分枝杆菌细胞包膜具有二层结构，由外菌膜、阿拉伯半乳聚糖-肽聚糖细胞壁、周质和内膜组成。脂甘露聚糖 (LM) 和脂阿拉伯甘露聚糖 (LAM) 是该细胞包膜的结构和免疫调节成分。LM/LAM 生物合成涉及许多甘露糖基转移酶和酰基转移酶，MptA 是一种 α1,6-甘露糖基转移酶，参与甘露聚糖链的最终延伸。最近，我们报道了参与耻垢分枝杆菌甘露聚糖骨架成熟的周质蛋白 LmeA 。在这里，我们检查了 LmeA 在压力条件下的作用。我们发现lmeA转录在两种胁迫条件下上调：稳定生长期和营养饥饿。在这两种情况下，LAM 都减少了，但 LM 相对稳定，这表明在压力下保持 LM 的细胞水平很重要。令人惊讶的是，在两种胁迫条件下，lmeA缺失 (Δ lmeA ) 突变体中 MptA 的蛋白质水平均降低。Δ lmeA突变体中mptA的转录水平与野生型相似甚至更高，表明 MptA 蛋白的减少是转录后事件。Δ lmeA突变体在压力下无法维持 LM 的细胞水平，这与 MptA 的降低一致。即使在活跃的生长过程中，LmeA 的过度表达也会导致细胞产生更多的 LM，并对几种抗生素产生更强的抵抗力。总之，我们的研究揭示了 LmeA 在 MptA 甘露糖基转移酶稳态中的作用，特别是在压力条件下，确保 LM 的稳定表达和细胞包膜完整性的维持。