Spread of Streptococcus pneumoniae from the nasopharynx to other host tissues would require the organism to adapt to a variety of environmental conditions. Since heat shock proteins are induced by environmental stresses, we investigated the effect of heat shock on ClpL and ClpP synthesis and the effect of clpL and clpP mutations on the expression of key pneumococcal virulence genes. Pulse labeling with [35 S]methionine and chase experiments as well as immunoblot analysis demonstrated that ClpL, DnaK, and GroEL were stable. Purified recombinant ClpL refolded urea-denatured rhodanese in a dose-dependent manner, demonstrating ClpL's chaperone activity. Although growth of the clpL mutant was not affected at 30 or 37°C, growth of the clpP mutant was severely affected at these temperatures. However, both clpL and clpP mutants were sensitive to 43°C. Although it was further induced by heat shock, the level of expression of ClpL in the clpP mutant was high at 30°C, suggesting that ClpP represses expression of ClpL. Furthermore, the clpP mutation significantly attenuated the virulence of S. pneumoniae in a murine intraperitoneal infection model, whereas the clpL mutation did not. Interestingly, immunoblot and real-time reverse transcription-PCR analysis demonstrated that pneumolysin and pneumococcal surface antigen A were induced by heat shock in wild-type S. pneumoniae. Other virulence genes were also affected by heat shock and clpL and clpP mutations. Virulence gene expression seems to be modulated not only by heat shock but also by the ClpL and ClpP proteases.

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Streptococcus pneumoniae usually colonizes the nasopharynx of humans asymptomatically but occasionally translocates from this niche to the lungs, the brain, and the blood, causing potentially fatal infections. Spread to other host tissues requires a significant morphological change and the expression of virulence factors, such as capsular polysaccharide, and virulence proteins, such as pneumolysin (Ply), PspA, and CbpA. Modulation of the expression of pneumococcal virulence genes by heat shock and by heat shock proteins ClpL and ClpP, as well as the attenuation of virulence of a clpP mutant in a murine intraperitoneal infection model, was demonstrated previously. In this study, we further investigated the underlying mechanism of virulence attenuation by the clpP mutation. The half-lives of the mRNAs of ply and of the first gene of the serotype 2 capsule synthesis locus [cps(2)A] in the clpP mutant were more than twofold longer than those of the parent after heat shock, suggesting that the mRNA species were regulated posttranscriptionally by ClpP. In addition, the clpP mutant was defective in colonization of the nasopharynx and survival in the lungs of mice after intranasal challenge. The mutant was also killed faster than the parent in the murine macrophage RAW264.7 cell line, indicating that ClpP is required for colonization and intracellular survival in the host. Furthermore, fractionation studies demonstrated that ClpP was translocated into the cell wall after heat shock, and immunization of mice with ClpP elicited a protective immune response against fatal systemic challenge with S. pneumoniae D39, making ClpP a potential vaccine candidate for pneumococcal disease.

Caseinolytic protease L (ClpL) is a member of the heat shock protein (Hsp) 100 family, which is found mostly in Gram‐positive bacteria. Here, ClpL, a major HSP in Streptococcus pneumoniae (pneumococcus), was biochemically characterized in vitro. Recombinant ClpL shows nucleotide hydrolase, refolding, holdase and disaggregation activity using either Mg2+ or Mn2+ and does not require the DnaK system for chaperone activity. ClpL exhibits two features distinct from other HSP100 family proteins: (a) Mn2+ enhances hydrolase activity, as well as chaperone activity; and (b) NTPase activity. ClpL forms a hexamer in the presence of ADP, ATP and ATP‐γ‐S. Mutational analysis using double‐mutant proteins mutated at the two Walker A motifs (K127A/T128A and K458A/T459A) revealed that both nucleotide‐binding domains are involved in chaperone activity, ATP hydrolase activity and hexamerization. Overall, pneumococcal ClpL is a unique Mn2+‐dependent Hsp100 family member that has chaperone activity without other co‐chaperones.

Antibiotic resistance and tolerance are increasing threats to global health as antibiotic-resistant bacteria can cause severe morbidity and mortality and can increase treatment cost 10-fold. Although several genes contributing to antibiotic tolerance among pneumococci have been identified, we report here that ClpL, a major heat shock protein, could modulate cell wall biosynthetic enzymes and lead to decreased penicillin susceptibility. On capsular type 1, 2, and 19 genetic backgrounds, mutants lacking ClpL were more susceptible to penicillin and had thinner cell walls than the parental strains, whereas a ClpL-overexpressing strain showed a higher resistance to penicillin and a thicker cell wall. Although exposure of Streptococcus pneumoniae D39 to penicillin inhibited expression of the major cell wall synthesis gene pbp2x, heat shock induced a ClpLdependent increase in the mRNA levels and protein synthesized by pbp2x. Inducible ClpL expression correlated with PBP2x expression and penicillin susceptibility. Fractionation and electron micrograph data revealed that ClpL induced by heat shock is localized at the cell wall, and the ⌬clpL showed significantly reduced net translocation of PBP2x into the cell wall. Moreover, coimmunoprecipitation with either ClpL or PBP2x antibody followed by reprobing with ClpL or PBP2x antibody showed an interaction between ClpL and PBP2x after heat stress. This interaction was confirmed by His tag pulldown assay with either ClpLHis 6 or PBP2xHis 6 . Thus, ClpL stabilized pbp2x expression, interacted with PBP2x, and facilitated translocation of PBP2x, a key protein of cell wall synthesis process, contributing to the decrease of antibiotic susceptibility in S. pneumoniae.