Abstract
Gas vesicles are gas-filled organelles that allow algae, bacteria and archaea to adjust their position in the water column for optimal illumination and aeration. The spindle-shaped vesicles are typically ∼500 nm long and ∼75 nm wide, with ∼1.9 nm thick walls and their shells consist exclusively of protein, primarily the highly hydrophobic GvpA monomer (70 residues), with a permeability such that the vesicle is filled with gas of atmospheric composition. Electron microscopy has shown that the GvpA monomers are arranged in a low-pitched helix; infrared spectroscopy shows considerable beta-sheet content, in agreement with results from X-ray scattering; and atomic force microscopy shows beta-strands tilted at an angle relative to the vesicle axis that is consistent with X-ray scattering measurements on partially aligned vesicles. To gain further insight into the molecular structure and interactions that grant gas vesicles their remarkable physical properties, atomic resolution data is required. However, insolubility prevents the use of solution NMR or crystallography, and multiple scattering frustrates high-resolution electron microscopy. Here, we present the results of solid-state NMR experiments aimed at characterizing the structure of GvpA in intact, deflated gas vesicles from Anabaena flos-aquae. Fairly complete dipolar correlation spectra, indicating a largely rigid and well-ordered system, have allowed resonance assignments for ∼80% of the protein sequence. These chemical shifts provide evidence for the presence of both beta-strand and alpha-helix elements in the GvpA backbone. Furthermore, certain regions of the sequence present duplicated resonances, which suggest that the basic structural subunit of gas vesicles is an asymmetric GvpA dimer. Finally, molecular mobility and preliminary tertiary structural characteristics are also discussed.