BlaR1 is a two-component signaling receptor that mediates antibiotic resistance in Staphylococcus aureus by sensing β-lactams and directly cleaving the repressor BlaI to induce expression of β-lactamase and β-lactam-resistant transpeptidase PBP2a. PDB ID: 8EXT. - Alexander et al. (2023), Nature.

We focus on preserving the efficacy of beta-lactam antibiotics, still the most prescribed class of antibiotics, by investigating and neutralizing the resistance mechanisms pathogens have acquired. Our lab uses advanced structural biology techniques, including x-ray crystallography and single-particle cryo-electron microscopy, to capture the first atomic-level insights into key resistance mechanisms in Staphylococcus aureus superbugs, such as MRSA, and highly resistant Enterobacteriaceae. These structural details have guided the development of novel, potent antibacterials in collaboration with the pharmaceutical industry, aiming to restore the effectiveness of beta-lactam antibiotics.

A primary focus of our work has been BlaR1, a receptor in MRSA that acts as a master switch responsible for turning on expression of multiple antibiotic resistance genes upon sensing beta-lactam drugs in the environment. In the clinically problematic MRSA strains, when BlaR1 senses antibiotics by acylation of its extracellular sensor domain, it changes conformation to send a signal across the membrane, setting off a chain of events that results in the production of PBP2a and PC1, enzymes that allows the bacteria to broadly and efficiently evade beta-lactam antibiotic treatment.

Using cryo-EM, our lab captured the first full length structure of BlaR1 in multiple states. We showed BlaR1 directly cleaves its own repressor protein BlaI, without help from other components, answering a long-standing question in the field about activation of this pathway. Additionally, we were able to identify a mechanism of signal transfer, showing the binding of drug displaces a key extracellular loop, driving a shift that switches the protease domain into an active, repressor cleaving state.

Because BlaR1 is the very beginning of this signaling cascade, disrupting its signaling offers a chance to shut down the entire signaling mechanism. Similar signal cascades have been identified in C. difficile and M. tuberculosis, suggesting relevance well beyond MRSA.

BlaR1's function as a drug sensor depends on other partners for its full function. A bacterial serine-threonine kinase, Stk1, phosphorylates BlaR1 to stabilize its position, a modification required for BlaR1's ability to detect β-lactams. Blocking Stk1 disrupts the process across most MRSA strains, restoring drug efficacy. We went on to determine the structural and kinetic characterization of an inhibitor bound Stk1, providing a starting point for Stk1 targeted anti-MRSA therapeutic.

Selected Publications