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The gas vesicles of the cyanobacteria Anabaena flos-aquae and Microcystis sp. contain, in addition to the principal M r 7400 gas vesicle protein GVPa, a second, M r 22000 protein component, GVPc. We show here that GVPc can be removed from the gas vesicles, without their collapsing, by rinsing in solutions of sodium dodecyl sulphate. It is concluded that GVPc is located on the outer surface of the hollow shell formed by GVPa. Removing GVPc causes a marked decrease in the critical collapse pressure of the gas vesicles. It is suggested that the protein provides structural support and reduces pressures generated by surface tension. By measurement of the 35S-labelling of Anabaena gas vesicles we demonstrate that GVPc, which contains two methionine residues per molecule, accounts for 9% of the total gas vesicle protein, confirming the value suggested by amino acid analysis. 32P-labelling indicates that the phosphate content of gas vesicle protein is negligible.
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