Sortase enzymes and their integral role in the development of Streptomyces coelicolor

Abstract

Sortase enzymes are cell wall-associated transpeptidases that facilitate the attachment of proteins to the peptidoglycan. Exclusive to Gram positive bacteria, sortase enzymes contribute to many processes, including virulence and pilus attachment, but their role in Streptomyces coelicolor biology remained elusive. Previous work suggested that the sortases anchored a subset of a group of hydrophobic proteins known as the long chaplins. The chaplins are important in aerial hyphae development, where they are secreted from the cells and coat the emerging aerial hyphae to reduce the surface tension at the air-aqueous interface. Two sortases (SrtE1 and SrtE2) were predicted to anchor these long chaplins to the cell wall of S. coelicolor. Deletion of both sortases or long chaplins revealed that although the long chaplins were dispensable for wild type-like aerial hyphae formation, the sortase mutant had a severe defect in growth. These two sortases were found to be nearly redundant, as deletion of individual enzymes led to only a modest change in phenotype. In vitro analysis of sortase cleavage activity showed that both sortases recognized the unique LAXTG pentapeptide sequence found in the long chaplins, and 11 other putative substrate proteins. Transcriptional analysis revealed that a number of genes typically expressed during aerial hyphae development were not expressed in a sortase deletion mutant. This suggests that the sortases have a role in transcriptional regulation, a phenomenon that has not been described previously. Current work is focused on addressing the mechanism(s) by which sortases affect transcriptional regulation, with a specific focus on the role of the proteins that they anchor to the cell wall (sortase substrates) in aerial growth.