Mechanistic studies of the Staphylococcus aureus cell division protein SmdA

Abstract

Understanding bacterial cell division is essential in the fight against bacterial infections and the current antibiotic resistance crisis. In Staphylococcus aureus, cell division must be carefully coordinated to successfully divide with consideration to its unique geometry. Many proteins in S. aureus have been directly and indirectly implicated in promoting correct division. One protein, named SmdA, was identified by our group and others to be essential for proper septal placement and resistance to antibiotics. However, no mechanism regarding how SmdA is able to promote successful cell division was proposed. In this work, I characterized SmdA both in vitro and in vivo. SmdA has a nuclease related domain, or NERD, and I showed that this domain has exonuclease activity. It cleaves single DNA in the 3’ to 5’ direction. To the best of my knowledge, no other NERD protein has been shown to have nuclease activity. I then used TnSeq to define ΔsmdA’s genetic interaction network and discovered that several proteins important for S. aureus cell division become essential when smdA is lost. Three of these proteins, EzrA, PcdA, and FacZ, interact directly or indirectly with FtsZ, a tubulin homolog that assembles in the presence of GTP to form the Z-ring that initiates cell division. I therefore hypothesized that SmdA may also interact with FtsZ. Consistent with this, a suppressor mutation in ftsZ, previously characterized in the Walker lab, corrects the cell division defects caused by loss of smdA. In silico modeling also predicts with high confidence that that SmdA and FtsZ form a complex. I conclude that SmdA directly regulates FtsZ dynamics to promote successful cell division. Although further work needs to be conducted to validate the proposed model for how SmdA regulates FtsZ dynamics, the findings in this work advance our understanding of SmdA and will ultimately lead to a better understanding of cell division in this critical pathogen.