Class A PBPs have a distinct and unique role in the construction of the pneumococcal cell wall.
Summary (2 min read)
Introduction
- The peptidoglycan layer covering the pneumococcal cell provides shape and rigidity, and is essential for growth and survival.
- Peptidoglycan is synthesized from lipid II precursors at the outside the cytoplasmic membrane by glycosyltransferases that polymerize the glycan chains and transpeptidases that interconnect the chains through peptide cross-links.
- So far the exact bond cleaved has not been identified.
- The SH3b domain is essential for the function of CbpD, and experimental evidence indicates that it binds to the peptidoglycan portion of the cell wall27.
- Hence, the authors have used the unique specificity of CbpD to study the functional relationships between different peptidoglycan synthesizing enzymes in S. pneumoniae.
Purification and properties of CbpD-B6
- The gene encoding cbpD from S. mitis B6 was amplified by PCR, ligated into the pRSET-A vector, and expressed using E. coli BL21 cells as a host.
- The recombinant CbpD-B6 protein was further purified by size-exclusion chromatography (SEC) on a SuperdexTM 75 10/300 GL column (see Methods section for details).
- CbpD-B6 attacks the septal area of the pneumococcal cell wall.
- The SEM microscopy analysis clearly showed that CbpD-B6 attacks only the septal region of the peptidoglycan sacculus, resulting in cells that are split in half along their equators (Fig. 2A and B).
- At the highest oxacillin concentrations used (50 and 100 µg ml-1), the pneumococci became as sensitive as untreated control cells (Fig. 3A).
The S2 phase
- During the S1-phases the oxacillin concentration increases gradually resulting in progressively stronger inhibition of PBP2x.
- The authors observed that the R-phase disappears if oxacillin (0.8 µg ml-1) and CbpD-B6 are added simultaneously to pneumococcal cultures.
- This finding suggested a plausible explanation for the S2-phase.
- The fact that pneumococcal cells need either PBP1a or PBP2a to survive, indicates that these PBPs can, at least to a certain extent, substitute for each other.
- In both cases the authors observed the usual S1, R and S2 phases (Fig. 4 and Fig. S5C and D), demonstrating that PBP2a can substitute for PBP1a in the process that transforms CbpD-B6-sensitive peptidoglycan into a resistant structure.
Discussion
- Recently it has become clear that FtsW/PBP2x and RodA/PBP2b constitute cognate pairs of interacting proteins that make up the core peptidoglycan synthesizing machineries within the pneumococcal divisome and elongasome, respectively9,10,11.
- This discovery has important implications for their understanding of pneumococcal cell wall synthesis, and the role played by class A PBPs in this process.
- The authors results show that class A PBPs act downstream of the FtsW/PBP2x machinery to produce alterations in the cell wall.
- This demonstrates that the elongasome is active even in the absence of a functional divisome.
- The authors clearly show that class A PBPs together with their associated auxiliary proteins somehow remodels the primary peptidoglycan synthesized by the PBP2x/FtsW machinery.
Methods
- All strains used in the present study are listed in Table S1.
- Liquid cultures were grown aerobically with shaking.
- Chemically competent E. coli cells were transformed by heat-shocking at 42ºC.
- Following incubation at 37°C for two hours, transformants were selected by plating 30 µl cell culture on TH-agar plates containing the appropriate antibiotic; kanamycin (400 µg ml-1), streptomycin (200 µg ml-1) or spectinomycin (200 µg ml-1).
DNA cloning
- All primers used in this study are listed in Table S2.
- The sf-gfp gene was amplified using the kp116 and kp119 primers and SPH370 genomic DNA as template, and the cbpD-B6-Δchap gene was amplified from SO7 genomic DNA using the primer pair kp117/kp118.
- The resulting sf-gfp-cbpD-B6 amplicon was cleaved with NdeI and HindIII and ligated into pRSET A giving the pRSET-sfGFP-cbpD-B6 plasmid.
- Amplicons used to transform S. pneumoniae were constructed by overlap extension PCR as previously described by Johnsborg et al.22.
- The spectinomycin resistant marker aad9 was employed to knock out lytA in strain ds789.
CbpD-B6 resistance assay
- Pneumococcal cells were grown in 96-wells microtiter plates and OD550 was measured every five minutes.
- The cells were grown for 10 minutes in the presence of antibiotics before purified CbpD-B6 was added to a final concentration of five µg ml-1. CbpD-sensitive cells were observed as a drop in OD550.
- Both cultures were incubated further for 10 minutes at 37°C before formaldehyde was added to a final concentration of 2.5%.
- Cells were then incubated in 100 µl PBS containing 0.05% Non-bound sfGFP-CbpD-B6 was washed off the cells by rinsing the glass slide by submerging the glass slide in five tubes each containing 40 ml PBS.
- Phase contrast pictures and GFP fluorescence pictures were captured using a Zeiss AxioObserver with an ORCA Flash4.0 V2 Digital CMOS camera (Hamamatsu Photonics) through a 100x PC objective.
Acknowledgments
- This work was supported by grants from the Research Council of Norway (no. 240058 and 250976) and the Norwegian University of Life Sciences.
- The percent reduction in cell density caused by cell lysis was determined.
- The experiments were repeated three times with the same results.
- Δpbp2a/Δpbp1b mutant (strain khb225) and Δpbp1a/Δpbp1b mutant (strain khb224) as well as a Δpbp2b mutant (strain ds789) all developed the typical S1, R, S2 phases upon increasing concentrations of oxacillin.
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Citations
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Cites background from "Class A PBPs have a distinct and un..."
...Nevertheless, it is evident that Class A PBPs play an important role in overall PG synthesis, being responsible for the major portion of PG in normal cells, crucial for width regulation, and modification of PG to a more mature form (Cho et al., 2016; Dion et al., 2019; Straume et al., 2020)....
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..., 2016), as well as modifying the PG to mature PG that is less sensitive to the activity of specific hydrolysases (Straume et al., 2020)....
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11 citations
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...The function of these two proteins is still not fully understood, however they are known to 294 be required for maturation of the cell wall as opposed to the construction of nascent peptidoglycan 295 (45)....
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5 citations
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...Earlier studies reported a high prevalence of tsst-1 gene among MSSA and β-lactam sensitive strains, but MRSA and β-lactam resistant strains typically do not and/ or less produce these superantigens (i.e. TSST-1, SEB, and SEC exotoxins)[19, 54]....
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...Both MRSA and MSSA can harbor one or more superantigenic toxin genes....
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...Skin and soft tissue infections (SSTIs) are commonly caused by S. aureus, specifically MRSA....
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...The pathogenic mechanism and virulence factors are assumed to be different between MRSA and MSSA [15]....
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References
126 citations
"Class A PBPs have a distinct and un..." refers methods in this paper
...pneumoniae were constructed by overlap extension PCR as previously described by Johnsborg et al.(22)....
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...Amplicons used to transform S. pneumoniae were constructed by overlap extension PCR as previously described by Johnsborg et al.22....
[...]
107 citations
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"Class A PBPs have a distinct and un..." refers methods in this paper
...The cells were lysed using the Fast Prep method with ≤106 µm glass beads at 6.5 m s−1 and insoluble material were removed by centrifugation at 20 000 × g. CbpD-B6 was purified from the soluble protein fraction by performing DEAE cellulose chromatography as described by Sanchez-Puelles et al.30, but using TBS (pH 7.4) instead of a phosphate buffer (pH 7.0)....
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...CbpD-B6 was purified from the soluble protein fraction by performing DEAE cellulose chromatography as described by Sanchez-Puelles et al.(30), but using TBS (pH 7....
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Frequently Asked Questions (2)
Q2. What future works have the authors mentioned in the paper "Class a pbps have a distinct and unique role in the construction of the pneumococcal cell wall" ?
Further confirmation or rejection of the repairosome hypothesis will be an important topic for future studies.