Dr Ian Jones
University of Reading, UK
The ESX-1 secretion-associated protein EspA may mediate liquid-liquid phase separation
The expression of EspA and EspC and secretion of the ESX-1 mediator proteins are mutually dependent (Fortune et al. 2005 ), although recent work suggests this dependence can be uncoupled (Cronin et al. 2022). Despite this the roles of each of the proteins themselves is uncertain. EspC has a tight helical structure which forms filamentous structures on the bacterial surface, plausibly as part of the secretion machinery (Lou et al. 2017) but the predicted structure of EspA shows the C-terminal half of the protein to be disordered and no discrete function has been reported. During a recent high throughput expression screen of multiple mycobacterial proteins we observed high level soluble expression of EspC but little or no expression of EspA unless it was constructed as a fusion protein with GFP (Paliwal et al. 2022). Expression of GFP-EspA in eukaryotic cells gave punctate rather than diffuse fluorescence. Neither mutation of the single cysteine in EspA, nor co-expression with EspC, EsxA or EsxB, altered the pattern of fluorescence. Deletion of the disordered C-terminus of EspA in the context of GFP-EspA produced a diffuse pattern of staining following expression consistent with a role for the disordered region in the observations made. When submitted to the FuzDrop server (Hatos et al. 2022) the EspA sequence scored highly for aggregation and liquid-liquid phase separation (LLPS) with the hotspots for each located in the disordered region. Interestingly several other proteins encoded by the RD1 locus, whose secretion is dependent on EspA/EspC, also scored highly suggesting they too may be capable of LLPS. As LLPS proteins are associated with the induction of a stress response and have been implicated in the pathogenicity of a number of diseases (Wang et al. 2021) these findings suggest a potential role for LLPS proteins in mycobacterial disease.
My degrees in Microbiology and Virology were obtained at the University of Warwick after which I worked as a PDRA in Paris and Oxford. Since establishing my own group I have been interested primarily in the structure and function relationships of viral structural proteins. This has involved considerable protein expression work which, more recently, we also applied to a range of M.bovis proteins, in order to provide a broader range of reagents for vaccine trials. Through this I have become interested in the properties of select M.bovis/Mtb proteins and in their novel configuration for diagnostic use.