Proteolytic regulation of the SsrA degradation motif in the human pathogen Staphylococcus aureus occurs through a seemingly unique pathway as compared to other prokaryotes. In most cases, the ATP- dependent protease ClpP recognizes and degrades SsrA-tagged proteins following recognition by either the chaperone ClpX or ClpA. S. aureus instead uses the chaperone ClpC and the adaptor TrfA to assist ClpP in SsrA degradation. This novel mechanism is further enhanced by the observation that SsrA breakdown continues to occur during thiol or oxidative stress in strains lacking TrfA, suggesting an additional layer of proteolytic regulation beyond ClpPC/TrfA. To fully characterize the regulation of SsrA-tagged protein levels in S. aureus, our first aim is to uncover additional regulators and adaptors of the SsrA degradation motif in S. aureus and determine their interplay with the known SsrA regulator TrfA. A sequence-defined transposon mutant library of S. aureus USA300 LAC JE2 will be transformed with a plasmid capable of expressing the rapidly maturing Venus protein that is C-terminally tagged with SsrA. To cast the widest net in this search, fluorescence patterns from strains exposed to a variety of stress conditions (e.g. diamide, starvation, etc.) will be obtained. Most strains in these populations will demonstrate low levels o fluorescence due to their persistent degradation of SsrA- tagged Venus. However, a small number will have mutations that affect SsrA degradation, which when compared to patterns from strains lacking clpC or trfA, will allow initial categorization into various control pathways. Subsequent strains doubly disrupted for each candidate gene and trfA will be created to further guide this process, as well as downstream complementation with each identified gene to hone a hierarchy of SsrA regulation in S. aureus. The use of ClpPC/TrfA in S. aureus as mediators of SsrA proteolysis suggests that the established motifs in SsrA used by other microbes may not be valid for S. aureus. Therefore, the S. aureus SsrA sequence may contain novel degradation signals, and our second aim will be to identify novel motifs in the SsrA tag, and to evaluate the importance of various residues in these motifs. Individual and groups of the eleven amino acids of SsrA appended to Venus will be altered to identify critical regions for proteolytic regulation. The resulting changes will be evaluated by monitoring fluorescence over time and during exposure to various stresses. Ultimately, unraveling how S. aureus regulates SsrA-tagged proteins provides an opportunity to better understand how this bacteria adjusts its proteome to challenging environments, and can provide S. aureus- specific targets for treating infections.
The human pathogen Staphylococcus aureus can be successfully transmitted from unexpected sources like clothing and gloves. It endures environmental damage and starvation in these locations by tightly controlling expression of unwanted proteins. One of the mechanisms it removes such proteins, via the ClpPC/TrfA system, is different from other known microbes, and better understanding of this mechanism will provide new opportunities to disrupt infections from S. aureus in the future.
