In methicillin-resistant Staphylococcus aureus, β-lactam antibiotic resistance is mediated by the transmembrane protein BlaR1. The antibiotic sensor domain BlaR(S) and the L2 loop of BlaR1 are on the membrane surface. We used NMR to investigate interactions between BlaR(S) and a water-soluble peptide from L2. This peptide binds BlaR(S) proximal to the antibiotic acylation site as an amphipathic helix. Acylation of BlaR(S) by penicillin G does not disrupt binding. These results suggest a signal transduction mechanism whereby the L2 helix, partially embedded in the membrane, propagates conformational changes caused by BlaR(S) acylation through the membrane via transmembrane segments, leading to antibiotic resistance.