Protein export systems of Mycobacterium tuberculosis: novel targets for drug development?

Abstract

Protein export is essential in all bacteria and many bacterial pathogens depend on specialized protein export systems for virulence. In Mycobacterium tuberculosis, the etiological agent of the disease tuberculosis, the conserved general secretion (Sec) and twin-arginine translocation (Tat) pathways perform the bulk of protein export and are both essential. M. tuberculosis also has specialized export pathways that transport specific subsets of proteins. One such pathway is the accessory SecA2 system, which is important for M. tuberculosis virulence. There are also specialized ESX export systems that function in virulence (ESX-1) or essential physiologic processes (ESX-3). The increasing prevalence of drug-resistant M. tuberculosis strains makes the development of novel drugs for tuberculosis an urgent priority. In this article, we discuss our current understanding of the protein export systems of M. tuberculosis and consider the potential of these pathways to be novel targets for tuberculosis drugs.

Figure 1. Barriers to effective drug delivery for Mycobacterium tuberculosis infection

M. tuberculosis is an intracellular pathogen that can reside long-term within macrophages in granulomas. Additionally, M. tuberculosis has an unusual and complex cell wall that is a physical barrier for drug delivery. The outermost layer of the cell wall is composed of a glycan- and glycolipid-based capsule, followed by the mycobacterial outer membrane. The mycobacterial outer membrane is composed of free lipids intercalated with mycolic acids that are covalently linked to the arabinogalactan component of the cell wall. The arabinogalactan is, in turn, covalently linked to peptidoglycan. The cytoplasmic membrane is the final permeability barrier before the cytosol.

Figure 2. The Sec, SecA2 and Tat protein export systems of mycobacteria

(A) The conserved Sec pathway exports unfolded preproteins that are synthesized with N-terminal signal peptides (hatched marks). The SecA ATPase recognizes and drives preproteins through the SecYEG membrane channel using repeated rounds of ATP-binding and hydrolysis. The SecD, SecF and YajC proteins increase efficiency of Sec protein export. During or shortly after translocation, the signal peptide is removed by a membrane-bound SP. The exported protein folds into a mature conformation. (B) The accessory SecA2 system of mycobacteria is characterized by a functionally distinct ATPase called SecA2, which is required for the export of a subset of proteins. Current data suggests that SecA2 uses the canonical SecA1/SecYEG translocase for exporting proteins, but the possibility of a requirement for a distinct channel within the membrane has not yet been eliminated. Some SecA2-dependent exported proteins have signal peptides and some do not. The role of SecA2 in the export of proteins lacking signal peptides is unresolved. (C) The conserved Tat pathway exports folded preproteins containing N-terminal signal peptides with a twin-arginine (RR) motif. Tat preproteins are recognized by the TatBC complex in the membrane and then translocated across a homo-oligomeric TatA channel. As with Sec export, the signal peptide is removed by a membrane-bound SP. RR: Arginine residues; Sec: Secretion; SP: Signal peptidase; tat: Twin arginine translocation.

Figure 3. The ESX-1 system of Mycobacterium tuberculosis

(A) The core components of the ESX-1 pathway are EccA1, EccB1, EccCa1, EccCb1, EccD1, EccE1 and MycP1. In the developing model of ESX export, C-terminal signal peptides of ESX-1 substrates (EspC and CFP-10) are recognized by cytoplasmic AAA ATPases (EccA1 and EccCb1). Through interactions with membrane components of the ESX-1 system, the ATPases are proposed to deliver these proteins to the translocation channel. EccD1 is the putative membrane channel through which the proteins are translocated. (B) The genomic ESX-1 locus of M. tuberculosis is shown along with the distally located espACD and espR genes. The RD1 deleted in BCG is indicated. (C) There is a recently introduced systematic nomenclature for genes of the ESX-1 locus [84]. The new gene names and previously used nomenclature, derived from the M. tuberculosis genome annotation, are shown. BCG: Bacille Calmette-Guérin; CFP: Culture filtrate protein; ESAT: Early secreted antigenic target; RD1: Region of difference 1.