Identification of Ligand Specificity Determinants in Lantibiotic Bovicin HJ50 and the Receptor BovK, a Multitransmembrane Histidine Kinase*

Background: Many lantibiotic peptides induce their own biosynthesis through histidine kinase receptors. Results: Bovicin HJ50 and BovK form a signaling complex; substitutions of key amino acids in each protein result in disrupted signal transduction. Conclusion: Bovicin HJ50 activates BovK through hydrophobic and electrostatic interaction to start signal transduction. Significance: A novel peptide activating multitransmembrane histidine kinase mechanism was identified. Lantibiotic bovicin HJ50 is produced by Streptococcus bovis HJ50 and acts as the extracellular signal to autoregulate its own biosynthesis through BovK/R two-component system. Bovicin HJ50 shows a linear N-terminal and glubolar C-terminal structure, and the sensor histidine kinase BovK contains eight transmembrane segments lacking any extensive surface-exposed sensory domain. The signal recognition mechanism between bovicin HJ50 and BovK is still unknown. We performed saturated alanine scanning mutagenesis and other amino acid substitutions on bovicin HJ50 using a semi-in vitro biosynthesis. Results of the mutants inducing activities indicated that several charged and hydrophobic amino acids in ring B of bovicin HJ50, as well as two glycines were key residues to recognize BovK. Circular dichroism analyses indicated that both glycines contributed to bovicin HJ50 structural changes in the membrane. Biotin-labeled bovicin HJ50 could interact with the N-terminal sensor of BovK, and several charged residues and a conserved hydrophobic region in the N-terminal portion of BovK sensor domain were important for interacting with the signal bovicin HJ50. By combining the results, we suggested a mechanism of bovicin HJ50 recognizing and activating BovK mainly through electrostatic and hydrophobic interactions.