1.C.95 The Pore-forming ESAT-6 Protein (ESAT-6) Family

The ESX-1 secretion system (TC# 9.A.25) plays a critical role in virulence of M. tuberculosis and M. marinum. Virulent M. marinum is able to escape from the Mycobacterium-containing vacuole (MCV) into the host cell cytosol, polymerize actin, and spread from cell to cell. ESX-1 plays an essential role in M. marinum escape from the MCV.

Smith et al. (2008) provided evidence that M. marinum induces membrane pores approximately 4.5 nm in diameter, and this activity correlates with ESAT-6 secretion. Purified ESTAT-6, but not the other ESX-1-secreted proteins, is able to cause dose-dependent pore-formation in host cell membranes.

M. tuberculosis uses protein secretion systems, named 6 kDa early secretory antigenic target (ESAT6) protein family secretion (ESX) systems to export a set of effector proteins that help the pathogen resist or evade the host immune response (Gröschel et al. 2016). Since the discovery of the esx loci during the M. tuberculosis H37Rv genome project, structural biology, cell biology and evolutionary analyses have been conducted. Gröschel et al. 2016 reviewed roles that these studies have revealed for ESX systems in bacterial survival and pathogenicity during infection with M. tuberculosis. They discuss the diversity of ESX systems that have been described among mycobacteria and selected non-mycobacterial species.

The reaction catalyzed for ESAT-6 is:

molecules (vacuoles) ⇌  molecules (cytoplasm)



Callahan, B., K. Nguyen, A. Collins, K. Valdes, M. Caplow, D.K. Crossman, A.J. Steyn, L. Eisele, and K.M. Derbyshire. (2010). Conservation of structure and protein-protein interactions mediated by the secreted mycobacterial proteins EsxA, EsxB, and EspA. J. Bacteriol. 192: 326-335.

Gröschel, M.I., F. Sayes, R. Simeone, L. Majlessi, and R. Brosch. (2016). ESX secretion systems: mycobacterial evolution to counter host immunity. Nat. Rev. Microbiol. 14: 677-691.

Nuñez-Garcia, J., S.H. Downs, J.E. Parry, D.A. Abernethy, J.M. Broughan, A.R. Cameron, A.J. Cook, R. de la Rua-Domenech, A.V. Goodchild, J. Gunn, S.J. More, S. Rhodes, S. Rolfe, M. Sharp, P.A. Upton, H.M. Vordermeier, E. Watson, M. Welsh, A.O. Whelan, J.A. Woolliams, R.S. Clifton-Hadley, and M. Greiner. (2018). Meta-analyses of the sensitivity and specificity of ante-mortem and post-mortem diagnostic tests for bovine tuberculosis in the UK and Ireland. Prev Vet Med 153: 94-107.

Smith, J., J. Manoranjan, M. Pan, A. Bohsali, J. Xu, J. Liu, K.L. McDonald, A. Szyk, N. LaRonde-LeBlanc, and L.Y. Gao. (2008). Evidence for pore formation in host cell membranes by ESX-1-secreted ESAT-6 and its role in Mycobacterium marinum escape from the vacuole. Infect. Immun. 76: 5478-5487.


TC#NameOrganismal TypeExample

Pore-forming ESAT-6 (EsxA) (95 aas) (Nuñez-Garcia et al. 2018). Secreted from the bacterial cytoplasm via a ESX protein secretion system (Type VII; TC# 3.A.24.5.1).


ESAT-6 of Mycobacterium tuberculosis (bovis) (P0A564).


Uncharacterized protein of 134 aas.

UP of Paracoccus aminophilus


Uncharacterized protein of 211 aas.

UP of Nakamurella lactea


Uncharacterized protein of 97 aas.

UP of Holdemania filiformis


Uncharacterized WXG100 family type VII secretion targetof 100 aas.

UP of Salinispora arenicola


ESAT-6-like protein, EsxA of 95 aas (Callahan et al. 2010).

ESAT-6-like protein of Corynebacterium diphtheriae


ESAT-6-like protein. a WXG100 (YukE) family type VII secretion target.

ESAT-6 protein of Gordonia rhizosphera


ESAT6-like protein of 95 aas.

ESAT-6-like protein of Amycolatopsis nigrescens


ESAT-6-like protein of 96 aas.

ESAT-6 of Intrasporangium chromatireducens


ESAT-6-like protein of 101 aas

UP of Nesterenkonia sp. F


ESAT-6-like protein of 103 aas.

ESAT-6-like protein of Williamsia marianensis


Uncharacterized protein of 96 aas

UP of Paeniglutamicibacter antarcticus


Uncharacterized WXG100 family type VII secretion target of 102 aas.

UP of Herbivorax saccincola