10EsxA has long been recognized as an important virulence factor of Mycobacterium tuberculosis 11 (Mtb) that plays an essential role in Mtb cytosolic translocation presumably by penetrating 12 phagosomal membranes with its acidic pH-dependent membrane permeabilizing activity (MPA). 13 However, current data suggest that the observed cytolytic activity of EsxA at neutral pH is due to 14 contamination of ASB-14, a detergent used in EsxA protein purification, and the role of EsxA 15 MPA in Mtb cytosolic translocation is also questionable. Here, we have obtained evidence that it 16 is ASB-14, not EsxA that causes cytolysis at neutral pH. Quantitative liquid chromatography and 17 mass spectrometry showed that even after gel filtration, dialysis, or passing through detergent 18 removal column, the remaining ASB-14 in the EsxA protein solution was still at a concentration 19 enough to kill cultured lung epithelial cells. When treated with trypsin or proteinase K, the digested 20 EsxA protein solution with ASB-14 was still cytotoxic. Interestingly, however, we have found that 21 the exogenously added EsxA is endocytosed into lung epithelial cells and inserts into the host 22 membranes within acidic subcellular compartments, which can be blocked by cytochalasin D and 23 bafilomycin A. It is for the first time EsxA is found to insert into the host membranes within acidic 24 subcellular compartments. 25 insertion 27 Importance: 28 EsxA has long been recognized as an important virulence factor of Mycobacterium tuberculosis 29 (Mtb) that plays an essential role in Mtb virulence. However, current data regarding to its role in 30 Mtb virulence are controversial. Here, we have obtained evidence showing that the cytolytic 31 activity of EsxA at neutral pH is due to contamination of ASB-14, a detergent used in EsxA 32 preparation. Moreover, it is for the first time we have found that EsxA protein is endocytosed into 33 lung epithelial cells and inserts into the host membranes within acidic subcellular compartments, 34 implicating an important role of the acidic pH-dependent membrane permeabilizing activity of 35 EsxA in Mtb virulence.36 37 38 39 2 Introduction 40 Mycobacterium tuberculosis (Mtb), the causative bacterial pathogen for tuberculosis disease, 41 infects one-third of the world's population and causes over one million deaths worldwide each 42year (1, 2). It is estimated that an airborne droplet carrying 1-3 Mtb bacilli is enough to cause 43 systemic dissemination and infection (3, 4). When Mtb is inhaled into the lung, it is engulfed into 44 alveolar macrophage. Instead of being destroyed in phagolysosomes like many other intracellular 45 pathogens, Mtb develops effective strategies to evade host defense mechanisms. Recent studies 46 have shown that Mtb is able to rupture the phagosomal membrane and translocate to the cytosol, 47 where it replicates, eventually lyses the macrophage and spreads to the surrounding non-48 phagocytic cells, such as alveolar endothelial and epithelial cells (5-10). Thus, cytosolic ...