Mutations in the major gas vesicle protein GvpA and impacts on gas vesicle formation in Haloferax volcanii.

Gas vesicles are proteinaceous, gas-filled nanostructures produced by some bacteria and archaea. The hydrophobic major structural protein GvpA forms the ribbed gas vesicle wall. An in-silico 3D-model of GvpA of the predicted coil-alpha 1-beta 1-beta 2-alpha 2-coil structure is available and implies that the two beta-chains constitute the hydrophobic interior surface of the gas vesicle wall. To test the importance of individual amino acids in GvpA we performed 85 single substitutions and analyzed these variants in Haloferax volcanii Delta A + A(mut) transformants for their ability to form gas vesicles (Vac(+) phenotype). In most cases, an alanine substitution of a non-polar residue did not abolish gas vesicle formation, but the replacement of single non-polar by charged residues in beta 1 or beta 2 resulted in Vac(-) transformants. A replacement of residues near the beta-turn altered the spindle-shape to a cylindrical morphology of the gas vesicles. Vac(-) transformants were also obtained with alanine substitutions of charged residues of helix alpha 1 suggesting that these amino acids form salt-bridges with another GvpA monomer. In helix alpha 2, only the alanine substitution of His53 or Tyr54, led to Vac(-) transformants, whereas most other substitutions had no effect. We discuss our results in respect to the GvpA structure and data available from solid-state NMR.