bioRxiv2024
Mycoplasma pneumoniae is a human pathogen causing atypical community-acquired pneumonia. It serves as a model for a minimal cell and is notable for its absent cell wall, minimal genome, and use of antigenic variation to evade the host immune response. Here, we report the structures of the essential surface proteins Mpn444 and Mpn436 at 3.74 [A] and 3.65 [A] resolution, and the molecular architecture of the Mpn444 homotrimeric complex. We show that both proteins include a peptidyl-prolyl isomerase (PPIase) domain and a chaperone-like domain. In vitro PPIase activity assays suggest that Mpn444 and Mpn436 function as extracellular foldases in Mycoplasma species. Furthermore, both proteins are conserved across multiple Mycoplasma species. We built a composite model integrating previously reported interactions from crosslinking and cryo-ET data and we conclude that Mpn444 is responsible for the extracellular folding of nascent protein chains. Our work underscores the potential of Mpn444 and Mpn436 as a target for the development of novel strategies to treat mycoplasma infections. Author SummaryIn this study, we discovered that the essential proteins Mpn444 and Mpn436 of Mycoplasma pneumoniae act as extracellular PPIases. We solved the structures of Mpn444, Mpn436, and Mpn444s homotrimer by single-particle cryo-EM analysis, marking them as the first experimentally resolved lipoproteins from Mycoplasma pneumoniae. The structures contain a peptidyl-prolyl isomerase (PPIase) domain as well as a chaperone-like domain, and the proteins show PPIase activity and bind unfolded proteins in vitro. By combining our structural data with previously published cryo-electron tomography and crosslinking data from M. pneumoniae cells, we suggest that Mpn444, and potentially Mpn436, works in concert with the Sec translocon and the membrane-bound ribosome. This finding is particularly important as the extracellular folding of bacterial proteins has been a significant gap in our understanding.