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Gas vesicles are gas-filled proteinaceous organelles that provide buoyancy for bacteria and archaea. A gene cluster that is highly conserved in various species encodes about 8-14 proteins (Gvp proteins) that are involved in the formation of gas vesicles. Here, the first crystal structure of the gas vesicle protein GvpF from Microcystis aeruginosa PCC 7806 is reported at 2.7 Å resolution. GvpF is composed of two structurally distinct domains (the N-domain and C-domain), both of which display an α+β class overall structure. The N-domain adopts a novel fold, whereas the C-domain has a modified ferredoxin fold with an apparent variation owing to an extension region consisting of three sequential helices. The two domains pack against each other via interactions with a C-terminal tail that is conserved among cyanobacteria. Taken together, it is concluded that the overall architecture of GvpF presents a novel fold. Moreover, it is shown that GvpF is most likely to be a structural protein that is localized at the gas-facing surface of the gas vesicle by immunoblotting and immunogold labelling-based tomography.

Staphylococcus aureus, a Gram-positive bacterium causes a number of devastating human diseases, such as infective endocarditis, osteomyelitis, septic arthritis and sepsis. S. aureus SraP, a surface-exposed serine-rich repeat glycoprotein (SRRP), is required for the pathogenesis of human infective endocarditis via its ligand-binding region (BR) adhering to human platelets. It remains unclear how SraP interacts with human host. Here we report the 2.05 Å crystal structure of the BR of SraP, revealing an extended rod-like architecture of four discrete modules. The N-terminal legume lectin-like module specifically binds to N-acetylneuraminic acid. The second module adopts a β-grasp fold similar to Ig-binding proteins, whereas the last two tandem repetitive modules resemble eukaryotic cadherins but differ in calcium coordination pattern. Under the conditions tested, small-angle X-ray scattering and molecular dynamic simulation indicated that the three C-terminal modules function as a relatively rigid stem to extend the N-terminal lectin module outwards. Structure-guided mutagenesis analyses, in addition to a recently identified trisaccharide ligand of SraP, enabled us to elucidate that SraP binding to sialylated receptors promotes S. aureus adhesion to and invasion into host epithelial cells. Our findings have thus provided novel structural and functional insights into the SraP-mediated host-pathogen interaction of S. aureus. [ABSTRACT FROM AUTHOR]