Expression, purification, and characterization of pneumococcal PsaA-PspA fusion protein

Streptococcus pneumoniae is a gram-positive bacterial pathogen causing invasive pneumonia, meningitis, otitis media, and bacteremia. Owing to the current pitfalls of polysaccharide and polysaccharide-conjugate vaccines, protein vaccines are considered promising candidates against pneumonia. Pneumococcal surface protein A (PspA) and pneumococcal surface adhesin A (PsaA) are virulence proteins showing good immunogenicity and protective effects against S. pneumoniae strains in mice. In this study, we expressed the fusion protein PsaA-PspA, which consists of PsaA and the N-terminal region of PspA family 1 and 2, in Escherichia coli. We describe a novel and effective method to purify PsaA-PspA using hydroxyapatite and two-step chromatography. After determining the optimal induction conditions and a series of purification steps, we obtained PsaA-PspA fusion protein with over 95% purity at a final yield of 22.44% from the starting cell lysate. The molecular weight of PsaA-PspA was approximately 83.6 kDa and its secondary structure was evaluated by circular dichroism. Immunization with the purified protein induced high levels of IgG antibodies in mice. Collectively, these results demonstrate that our purification method can effectively produce high-purity PsaA-PspA fusion protein with biological activity and chemical integrity, which can be widely applied to the purification of other PspA subclass proteins.