Proton gradient controls the lateral rearrangement of inner membrane domains in response to membrane fluidizer stress in Mycobacterium smegmatis

Mycobacterium smegmatis partitions its plasma membrane into two distinct regions: the inner membrane domain (IMD) and the conventional plasma membrane. IMD, enriched in the sub-polar regions of actively growing rod-shaped cells, contains many membrane proteins involved in cell envelope biosynthesis. Dibucaine, a membrane fluidizer, disrupts plasma membrane integrity and de-partitions the IMD from the subpolar regions. We do not know what governs the de-partitioning of the IMD in response to dibucaine stress. In this study, we investigated the stress response of the IMD under respiration defect. We first depleted MenG, a key enzyme in the menaquinone biosynthesis, by CRISPRi and observed that the IMD does not respond to dibucaine-induced membrane stress. CRISPRi-induced knockdown of qcrC, a gene encoding a component of an electron transport chain cytochrome, corroborated the results of menG knockdown. In contrast, neither CRISPRi knockdown of atpD, a gene encoding a component of the ATP synthase nor inhibition of ATP synthase by bedaquiline inhibited the dibucaine-induced de-partitioning of sub-polar IMD as robustly as CRISPRi knockdowns of menG and qcrC. Pretreatment with the protonophore carbonyl cyanide m-chlorophenyl hydrazone (CCCP) prevented dibucaine-induced IMD de-partitioning. Furthermore, pretreatment with nigericin, which acts as an H⁺/K⁺ antiporter and disrupts the proton gradient without affecting membrane potential, also inhibited the IMD de-partitioning in a way similar to CCCP. Taken together, our findings suggest that membrane stress-induced IMD delocalization is not a passive lipid dispersion but an active membrane rearrangement dependent on an electrochemical gradient of the proton.