Analysis of Streptococcal Accessory Secretion System and Novel Secreted Proteins

The bulk of protein secretion in bacteria is via the general secretion (Sec) pathway. Proteins are led to the Sec pathway by a typical N-terminal signal peptide, where they undergo processing and folding within the cell wall environment. Gram-positive bacteria have been shown to possess more than one Sec system. It appears that the alternate system, known as the accessory secretion system, may secrete a specific set of extracellular proteins, some of which lack a typical N-terminal leader peptide. The core component of the accessory Sec system in Gram-positive bacteria is SecA2 protein, a homologue of SecA from the canonical Sec system. Objectives: This study aimed to determine the specificity of accessory SecA2 system in Streptococcus gordonii and S. pneumoniae for secretion of proteins with a conventional or non-conventional signal peptide. Methods: Knockout secA2 mutants were generated and used to investigate the role of SecA2 in protein secretion, adhesion and biofilm formation. Results: Adherence assays and mass spectrometry analysis confirmed that known SecA2-dependent surface proteins Hsa (S. gordonii) and PsrP (S. pneumoniae) were not secreted when secA2 was inactivated. Haemolytic assays suggested that SecA2 may be involved in release of pneumolysin, a S. pneumoniae toxin lacking a signal peptide. Additionally, biofilm formation was notably reduced in both wild-type strains compared to ΔsecA2 mutants. Conclusion: As expected, cell wall anchored adhesins Hsa and PsrP required SecA2 for secretion. The accessory secretion system may also facilitate secretion of pneumolysin. SecA2 was implicated in biofilm formation, a phenotype often associated with bacterial colonisation and pathogenic capabilities.