Laboratory of Cell Regulation, Institute for Virus Research, Kyoto University, Kyoto 606-8502, Japan.
J Biochem. 2009 Nov;146(5):659-65. doi: 10.1093/jb/mvp113. Epub 2009 Jul 23.
Trehalose dimycolate (TDM) is a major surface-exposed mycolyl glycolipid that contributes to the hydrophobic cell wall architecture of mycobacteria. Nevertheless, because of its potent adjuvant functions, pathogenic mycobacteria appear to have evolved an evasive maneuver to down-regulate TDM expression within the host. We have shown previously that Mycobacterium tuberculosis (M.tb) and Mycobacterium avium (M.av), replace TDM with glucose monomycolate (GMM) by borrowing host-derived glucose as an alternative substrate for the FbpA mycolyltransferase. Mycobacterium leprae (M.le), the causative microorganism of human leprosy, is also known to down-regulate TDM expression in infected tissues, but the function of its mycolyltransferases has been poorly analysed. We found that, unlike M.tb and M.av FbpA enzymes, M.av FbpA was unexpectedly inefficient in transferring alpha-branched mycolates, resulting in impaired production of both TDM and GMM. Molecular modelling and mutational analysis indicated that a bulky side chain of leucine at position 130 of M.le FbpA obstructed the intramolecular tunnel that was proposed to accommodate the alpha-branch portion of the substrates. Notably, even after a highly reductive evolution, M.le FbpA remained functional in terms of transferring unbranched acyl chains, suggesting a role that is distinct from that as a mycolyltransferase.
海藻糖二分枝菌酸酯（TDM）是一种主要的表面暴露的菌酸甘油酯，有助于分枝杆菌的疏水性细胞壁结构。然而，由于其强大的佐剂功能，致病性分枝杆菌似乎已经进化出一种逃避策略，在宿主中下调 TDM 的表达。我们之前已经表明，结核分枝杆菌（M.tb）和鸟分枝杆菌（M.av）通过借用宿主来源的葡萄糖作为 FbpA 酰基转移酶的替代底物，用葡萄糖单分枝菌酸酯（GMM）替代 TDM。麻风分枝杆菌（M.le），人类麻风病的致病微生物，也被认为在感染组织中下调 TDM 的表达，但对其酰基转移酶的功能分析甚少。我们发现，与 M.tb 和 M.av FbpA 酶不同，M.av FbpA 出乎意料地不能有效地转移α-支链菌酸酯，导致 TDM 和 GMM 的产量都受到损害。分子建模和突变分析表明，M.le FbpA 位置 130 的亮氨酸的大侧链阻碍了拟议容纳底物α-支链部分的分子内隧道。值得注意的是，即使在高度还原进化后，M.le FbpA 在转移无支链酰基链方面仍然具有功能，表明其作用不同于酰基转移酶。
