Biochemical and structural characterization of a putative chaperone protein that mediates secretion of antibacterial toxins in Gram-positive bacteria

Abstract

Bacteria live in complex and densely populated environments where they engage in antagonistic interactions with other bacteria for access to various resources. In addition to antibiotics, one of the most powerful mechanisms bacterial have developed to eradicate their competitors is the use of membrane-embedded macromolecular machines, called secretion systems, which inject antibacterial protein toxins directly into neighboring cells inhibiting their growth. Many Gram-positive pathogenic bacteria use the Type VII Secretion System to export a unique family of proteins known as LXG toxins. While the toxic functions of many LXG proteins are known or can be predicted, the mechanism by which these toxins are secreted is poorly understood and of critical importance to the field. I recently discovered a novel protein of unknown function that is widely distributed among LXG toxin-producing Gram-positive bacteria. Experiments performed using wild-type and gene knockouts in Streptococcus show that the presence of this protein in the cell is required for the secretion of LXG toxins. Biochemical experiments reveal that this protein physically interacts with a region of unknown function within the LXG toxin. I propose that this protein functions as a molecular chaperone that maintains the stability and solubility of the LXG toxin prior to its secretion from the cell. Lastly, we determined the X-ray crystal structure of this putative chaperone both in an unbound state and in complex with its toxin binding partner and identified structural features important for its function. Identifying the key players implicated in the process of toxin secretion advances our overall knowledge on one of the most important mechanisms bacteria have developed to survive. A deeper understanding of how complex microbial communities are shaped will allow for the development of new strategies that seek to manipulate pathogenic and commensal bacterial populations in a way that is beneficial to human health.