Effect of calcium on the in vivo assembly of the surface protein of Aquaspirillum serpens VHA

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Abstract
Aquaspirillum serpens VHA grown in minimal medium supplemented with 0.5 mM calcium possesses the regularly structured (RS) superficial protein layer. Cells grown in calcium-deficient medium lack the RS layer, but secrete small amounts of RS protein into the culture supernatant. Thus calcium is not required for synthesis of this protein. Addition of 0.5 mM calcium to midexponential phase cells in calcium-deficient conditions results in the rapid (within 10 min) assembly of a stable RS layer on the outer membrane. The RS layer is assembled from secreted and newly synthesized protein. No periplasmic pool of RS protein was found in calcium-deficient conditions. It is suggested that calcium is required to establish the correct conformation of the protein to allow assembly.