Structural insights into loss of function of a pore forming toxin and its role in pneumococcal adaptation to an intracellular lifestyle.

• Main Authors: Dilip C Badgujar, Anjali Anil, Angharad E Green, Manalee Vishnu Surve, Shilpa Madhavan, Alison Beckett, Ian A Prior, Barsa K Godsora, Sanket B Patil, Prachi Kadam More, Shruti Guha Sarkar, Andrea Mitchell, Rinti Banerjee, Prashant S Phale, Timothy J Mitchell, Daniel R Neill, Prasenjit Bhaumik, Anirban Banerjee
• Format: Article
• Language: English
• Published: Public Library of Science (PLoS) 2020-11-01
• Series: PLoS Pathogens
• Online Access: https://doi.org/10.1371/journal.ppat.1009016
• ISSN: 1553-7366; 1553-7374

The opportunistic pathogen Streptococcus pneumoniae has dual lifestyles: one of an asymptomatic colonizer in the human nasopharynx and the other of a deadly pathogen invading sterile host compartments. The latter triggers an overwhelming inflammatory response, partly driven via pore forming activity of the cholesterol dependent cytolysin (CDC), pneumolysin. Although pneumolysin-induced inflammation drives person-to-person transmission from nasopharynx, the primary reservoir for pneumococcus, it also contributes to high mortality rates, creating a bottleneck that hampers widespread bacterial dissemination, thus acting as a double-edged sword. Serotype 1 ST306, a widespread pneumococcal clone, harbours a non-hemolytic variant of pneumolysin (Ply-NH). Performing crystal structure analysis of Ply-NH, we identified Y150H and T172I as key substitutions responsible for loss of its pore forming activity. We uncovered a novel inter-molecular cation-π interaction, governing formation of the transmembrane β-hairpins (TMH) in the pore state of Ply, which can be extended to other CDCs. H150 in Ply-NH disrupts this interaction, while I172 provides structural rigidity to domain-3, through hydrophobic interactions, inhibiting TMH formation. Loss of pore forming activity enabled improved cellular invasion and autophagy evasion, promoting an atypical intracellular lifestyle for pneumococcus, a finding that was corroborated in in vivo infection models. Attenuation of inflammatory responses and tissue damage promoted tolerance of Ply-NH-expressing pneumococcus in the lower respiratory tract. Adoption of this altered lifestyle may be necessary for ST306 due to its limited nasopharyngeal carriage, with Ply-NH, aided partly by loss of its pore forming ability, facilitating a benign association of SPN in an alternative, intracellular host niche.