The fibronectin binding protein PavA is involved in ribosome quality control in Streptococcus pneumoniae

Introduction The pneumococcal adherence and virulence factor PavA was first described as virulence factor and fibronectin-binding protein (Kanwal et al 2017). The deficiency of PavA in pneumococcal mutants abrogates adherence to human host cells and impairs mouse colonization. Moreover, PavA is required to escape phagocytosis and needed for induction of an optimal cytokine production by infected dendritic cells (Noske et al 2009). However, the biological activity associated with its crucial role in maintaining virulence remains unknown.

Methods In silico analysis and protein sequence comparisons revealed significant homology to the ribosome quality control protein RqcH of B. subtilis. The RqcH protein is part of the ribosome quality control system (RQC) recognizing stalled 50S ribosomes subunits and promotes nascent chain proteolysis. To study the involvement of PavA as part of the RQC system, protein-protein interaction was analyzed between PavA and RqcP.

Results In a pull down experiment we were able to identify binding of PavA to RqcP. Surface Plasmon Resonance (SPR) analysis revealed a significant affinity of PavA to RqcP with a Kd ranging from 0.12 to 0.22 µM depending on the provided ligand. Growth behavior of D39 and 19F strains and the isogenic pavA mutants were studied. No growth difference was detected between the wild-type and the pavA mutant during cultivation in THY or minimal medium. Furthermore, the impact of PavA on oxidative stress reduction was investigated. So far no significant difference between the wild-type and the isogenic pavA-mutant was observed when hydrogen peroxide or diamide was applied as oxidative agents. In addition, RqcP as part of the ribosome was mutated and the growth of single and double pavA, rqcP deletion were studied.

Summary The striking homology of PavA to RqcH of B. subtilis and the protein interaction suggests that PavA is part of the ribosome quality control in S. pneumoniae. In vitro assays even under infection-related stress conditions did not result in growth defects so far. Hence, the stress conditions in which PavA (RqcH) is essential for pneumococci have still to be elucidated.