Mycobacterium tuberculosis effector proteins: functional multiplicity and regulatory diversity

Tuberculosis (TB), which is caused by Mycobacterium tuberculosis (Mtb), remains the leading cause of death from a single infectious agent. Moreover, TB is responsible for approximately one-quarter of all deaths related to antimicrobial resistance upon the emergence of drug-resistant TB.¹ Mtb infection is also associated with multiple other human diseases, such as lung cancer, autoimmune diseases, and metabolic syndromes. Because Mtb lacks classical bacterial toxins (including endotoxins and exotoxins), its pathogenesis mainly involves intricate pathogen-host interplay, which is mediated by a variety of pathogenic factors, including surface-associated and secreted effector proteins. Therefore, a better understanding of the molecular details of the Mtb–host interaction is invaluable for the development of new therapeutic strategies against TB.

Among numerous Mtb effector proteins, mammalian cell entry (Mce) family proteins have been implicated in manipulating multiple host cell signaling pathways.² For example, Mce3C was identified as an Mtb surface protein that interacts with β2 integrin to trigger SFK-Syk-Vav-Rho-ROCK cascade signaling, leading to enhanced pathogen invasion of macrophages.³ Another study reported that Mce3E, through its DEF motif, suppresses the host ERK1/2 signaling pathway, leading to innate immune suppression and increased pathogen intracellular survival.⁴ Interestingly, a more recent study by Qiang and colleagues revealed that Mce2E exerts dual host-regulatory functions, including suppressing macrophage innate immune responses and promoting epithelial cell proliferation in a niche-dependent manner.⁵ As shown in this study, the dual functions of Mce2E depend upon its different domains. On the one hand, Mce2E inhibits ERK and JNK signaling activation by binding to ERK2 and JNK1 via its noncanonical D motif, which is different than classical MAPK-docking motifs and the DEF motif in the Mce3E protein. On the other hand, by binding to host eEF1A1 (eukaryotic translation elongation factor 1A1) via its β-strand region in epithelial cells, Mce2E inhibits the K48-linked polyubiquitination-mediated proteasomal degradation of the eEF1A1 protein to promote tumor cell proliferation and tumor growth in vivo. Thus, through different domains, Mce2E regulates host innate immune responses and carcinogenesis in a cell-specific manner.

In addition to Mce family proteins, a variety of other Mtb effector proteins also possess multifunctional domains and cell-type-specific regulatory modes, which are beneficial for the pathogen to manipulate multiple host cellular signaling pathways and processes, such as immune and inflammatory signaling pathways, autophagy, cell proliferation, and cell death (Fig. 1). One example is the tyrosine phosphatase PtpA, a secreted Mtb effector protein that is critical for TB pathogenicity, which suppresses innate immunity by hijacking the host ubiquitin system. Specifically, PtpA binds to ubiquitin via a previously unknown ubiquitin-interacting motif-like domain, which is in turn activated and dephosphorylates p-JNK, p-p38, and VPS33B (vacuolar protein sorting 33B) to suppress JNK and p38 signaling pathways and to inhibit macrophage phagosome–lysosome fusion and thus inhibit pathogen clearance.⁶ A further follow-up study of PtpA showed that PtpA enters the nucleus of host cells and regulates the expression of host genes by binding to target DNA in a phosphatase-independent manner. PtpA can directly bind to the promoter region of host GADD45A (growth arrest and DNA damage-inducible alpha) to promote the proliferation and migration of A549 human lung adenoma cells in vitro and in a mouse xenograft model.⁷ In addition, both the Mtb-secreted proteins MmsA and ESAT-6 can promote dendritic cell activation depending on multiple signaling pathways, including the NF-κB and MAPK pathways.^8,9 Interestingly, MmsA and ESAT-6 have distinct effects on macrophages. Basically, MmsA inhibits type-I IFN signaling by targeting STING (stimulator of interferon genes) for p62-mediated autophagic degradation, whereas ESAT-6 interacts with β2M (beta-2-microglobulin) to inhibit macrophage-dependent antigen presentation.^10,11 Furthermore, Mtb PPE37 was reported to be cleaved by an Mtb protease into N- and C-terminal segments that have opposite effects on the differentiation, proliferation and apoptosis of macrophages.¹² Notably, an increasing number of studies have revealed that multifunctional effector proteins also serve as versatile weapons for other pathogenic bacteria in their battles against host immune surveillance. For instance, Helicobacter pylori virulence factor VacA can interact with T-cell surface protein β2 integrin to inhibit T-cell activation and proliferation. VacA also prevents phagosome–lysosome fusion in macrophages by recruiting the host coat protein TACO.¹³ Another study revealed that Salmonella enterica SseL exhibits cell-type-specific functions including the regulation of macrophage cell death and a reduction in lipid droplet accumulation in epithelial cells.¹⁴

Fig. 1.

Summarization of Mtb multifunctional effector proteins that regulate host cellular processes in a cell-type-specific manner. Mce2E and PtpA inhibit macrophage immune signaling pathways and promote epithelial cell proliferation through different mechanisms. ESAT-6 interacts with host β2M cells to inhibit the antigen presentation function of macrophages, and it also promotes dendritic cell activation through TLR-2- and MyD88-dependent NF-κB and MAPK signaling pathways. Another Mtb effector protein, MmsA, induces dendritic cell activation depending on the NF-κB and MAPK signaling pathways while inhibiting the type-I IFN response by targeting STING for p62-mediated autophagic degradation

Genome sequencing revealed that Mtb H37Rv harbors 4056 genes, including 3995 predicted proteins,¹⁵ the majority of which have host functions and regulatory mechanisms that remain largely unknown. Thus, new techniques and approaches are warranted to provide more in-depth insights into the functional multiplicity and regulatory diversity of Mtb effector proteins within host cells. The dilemma of drug-resistant TB and the unsatisfying protective effects of BCG vaccines have revealed that the traditional design of drugs based on single pathogen targets has limitations in treating TB since this strategy unavoidably causes drug resistance. The recently emerging concept of host-directed therapy that targets host processes to limit mycobacterial infection may have an increased risk of unwanted side effects. Thus, targeting specific Mtb–host interfaces may provide a more rational alternative strategy for TB treatment since it would minimize the emergence of drug resistance and unwanted side effects. Furthermore, given the diverse host-regulatory functions of Mtb effector proteins, considering multiple targets for TB treatment may lead to more promising effects.

Competing interests

The authors declare no competing interests.