Secreted via the ESX-1 / type VII secretion system (T7SS). The C-terminus is necessary for the co-dependent secretion, for maintaining the EsxA (ESAT-6) cellular levels, and for interacting with EsxA. Cleaved at close to the C-terminus during secretion. Mutant is extremely defective in intracellular growth. Secretion of EspB, EsxA and EsxB is mutually dependent. Belongs to the EspB family.