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(l to r) Martha Zepeda Rivera, Briana Burton, and Tanya Sysoeva

One way that bacteria communicate with each other is by exporting proteins from the cytoplasm into the surrounding media. All described bacterial protein secretion systems recognize and export one protein at time. Recently published work by Sysoeva et al. in the Burton lab describes how a new family of secretion systems (called ESX) recognize and export a pair of proteins (Figure).

A. Newly identified features of the ESX secretory mechanism (middle) differ from the major secretory pathways – general secretory pathway (Sec) and twin-arginine translocon (Tat). B. Model depicting the two-factor identification signals (in red) of a dimeric ESX substrate.

The ESX protein secretion system is critical for the virulence of the deadly human pathogens Staphylococcus aureus and Mycobacterium tuberculosis.

Laura Huppert, an undergraduate in the lab, first developed methods to study a model of ESX secretion in the non-pathogenic bacterium, Bacillus subtilis. This work demonstrated that the ESX system in B. subtilis is functional and responsible for export of a virulence factor homolog called YukE (Huppert et al., 2014). In B. subtilis YukE is the only detected ESX secretion substrate, therefore the B. subtilis system presented a unique opportunity to determine how the ESX machinery recognizes and exports its proteins out of the cell.

Sysoeava’s work published this week in PNAS, revealed that ESX export requires two independent export motifs: the carboxy-terminal tail of the YukE protein, as well as a short stretch of amino acids (residues WXG) in the middle of the protein. Like other ESX substrates YukE forms stable head to tail dimers. Coexpressing different substrate dimers, the group showed that two recognition motifs must come together to form a composite secretion signal in which one motif is contributed by each subunit of the dimer. This is the first demonstration of such a two-component signal required for protein secretion.

The authors then asked whether the composite signal presented by the dimer of YukE meant that that dimer might be exported as an intact complex. To do this, they effectively “locked” the dimer together using a targeted irreversible crosslinking scheme. The result was that the locked dimer that had been formed inside the cell was indeed exported by the ESX machinery. This is the first example of a higher order protein complex being exported from a cell.

The authors’ observations lead to a simple model in which two-factor authentication allows for pairs of substrates to be selected for export. The simplicity of the recognition system provides a unifying model to explain the observation that a large number of specific heterodimeric pairs may be exported by the mycobacterial ESX systems. The double identification hypothesis might also explain how several coexisting mycobacterial ESX secretory pathways manage to regulate and fine tune secretion of tens of similar substrates.

Read more in PNAS or download PDF.

The Burton lab in collaboration with Sarah Fortune at Harvard School of Public Health have additional studies testing the features of the secretion machinery that are common between the B. subtilis and M. tuberculosis systems.

Martha Zepeda-Rivera is a Howard Hughes Medical Institute Gilliam fellow.

This work was supported in part by the William F. Milton Fund to Briana Burton.

Huppert L.A., Ramsdell T.L., Chase M.R., Sarracino D.A., Fortune S.M., Burton B.M. (2014) The ESX System in Bacillus subtilis Mediates Protein Secretion. (2014) PloS one, DOI:10.1371/journal.pone.0096267

Sysoeva T.A., Huppert L.A, Zepeda-Rivera M.A., Burton B.M. Dimer recognition and secretion by the ESX Secretion System in Bacillus subtilis. (2014) Proc Natl Acad Sci USA. (in press)