Required for ESX-1 function. Required for either stability or expression of EspA. Interacts with EccA1 and EspF. Secreted via the ESX-1 / type VII secretion system (T7SS). Transcriptionally activated by EspR (PubMed:18685700, PubMed:22389481, PubMed:25536998). Repressed by the MprB/MprA two-component system, by direct regulation and via EspR (PubMed:23104803, PubMed:25536998). Up-regulated by the PhoP/PhoR two-component system, via EspR (PubMed:16573683, PubMed:25536998). The C-terminal region is required for secretion by the ESX-1 system. Mutant exhibits pathogenesis defects, including reduced virulence in mice, attenuated growth in macrophages and an inability to suppress immunostimulatory cytokines such as IL-12 (PubMed:16135231). Host (human) cells no longer produce cytokine IP-10 (CXCL10) upon infection, but continue to produce IL-1 beta (IL1B) (PubMed:26048138). Contains multiple, broadly recognized T-cell epitopes and is a major target of cellular immunity in both active and latent tuberculosis infection, indicating this might be a good vaccine candidate (PubMed:21427227). A fusion protein (p846) of 3 well-defined antigens (EspC, EsxH and Rv2660c) induces robust specific T-cell immune response and could be an effective vaccine (PubMed:24280763). Belongs to the EspC family.