期刊名称：Proceedings of the National Academy of Sciences
印刷版ISSN：0027-8424
电子版ISSN：1091-6490
出版年度：2022
卷号：119
期号：33
DOI：10.1073/pnas.2203156119
语种：English
出版社：The National Academy of Sciences of the United States of America
摘要：Significance
Deinococcus radiodurans is an extremophilic bacterium that has been studied intensely due to its hyperstability and deep position in the evolutionary tree of life, respectively. An atypical cell envelope is one factor underlying its hyperstability; however, molecular organizational principles central to this envelope remain unclear. We have solved the atomic structure of a highly abundant protein, SlpA, and discovered that it forms extended structures that link the outer membrane (OM) to the peptidoglycan (PG). SlpA-like putative OM–PG connector proteins are widely present in many Gram-negative phyla, where they likely play a key role in organizing the bacterial cell envelope. Our results will have important implications for understanding the organization and evolution of bacterial cell surfaces.
Deinococcus radiodurans is a phylogenetically deep-branching extremophilic bacterium that is remarkably tolerant to numerous environmental stresses, including large doses of ultraviolet (UV) radiation and extreme temperatures. It can even survive in outer space for several years. This endurance of
D. radiodurans has been partly ascribed to its atypical cell envelope comprising an inner membrane, a large periplasmic space with a thick peptidoglycan (PG) layer, and an outer membrane (OM) covered by a surface layer (S-layer). Despite intense research, molecular principles governing envelope organization and OM stabilization are unclear in
D. radiodurans and related bacteria. Here, we report a electron cryomicroscopy (cryo-EM) structure of the abundant
D. radiodurans OM protein SlpA, showing how its C-terminal segment forms homotrimers of 30-stranded β-barrels in the OM, whereas its N-terminal segment forms long, homotrimeric coiled coils linking the OM to the PG layer via S-layer homology (SLH) domains. Furthermore, using protein structure prediction and sequence-based bioinformatic analysis, we show that SlpA-like putative OM–PG connector proteins are widespread in phylogenetically deep-branching Gram-negative bacteria. Finally, combining our atomic structures with fluorescence and electron microscopy of cell envelopes of wild-type and mutant bacterial strains, we report a model for the cell surface of
D. radiodurans. Our results will have important implications for understanding the cell surface organization and hyperstability of
D. radiodurans and related bacteria and the evolutionary transition between Gram-negative and Gram-positive bacteria.
