Identification of an osmotically induced periplasmic glycine betaine-binding protein from Rhizobium meliloti.

The effect of salt stress on glycine betaine-binding activity has been investigated in periplasmic fractions released from Rhizobium meliloti 102F34 by cold osmotic shock. Binding activity was monitored by three techniques: equilibrium dialysis, filter procedure, and detection of 14C ligand-protein binding by direct non-denaturing polyacrylamide gel electrophoresis (PAGE) followed by autoradiography. The three methods demonstrated the existence of a strong glycine betaine-binding activity, but only in periplasmic fractions from cells grown at high osmolarity. The non-denaturing PAGE of such periplasmic shock fluids mixed with [methyl-14C]glycine betaine showed only one radioactive band, indicating the involvement of one glycine betaine-binding protein. To determine the possible implication of this binding protein in glycine betaine uptake, transport activity was measured with cells submitted to cold osmotic shock. No significant decrease of transport activity was noticed. This lack of effect could be explained by the small quantity of periplasmic proteins released as judged by the low activity of phosphodiesterase, a periplasmic marker enzyme, observed in the shock fluid. The specificity of binding was analysed with different potential competitors: other betaines such as gamma-butyrobetaine, proline betaine, pipecolate betaine, trigonelline and homarine, or amino acids like glycine and proline, did not bind to the glycine betaine-binding protein, whereas glycine betaine aldehyde and choline were weak competitors. Optimum pH for binding was around 7.0, but approx. 90% of the glycine betaine-binding activity remained at pH 6.0 or 8.0. The calculated binding affinity (KD) was 2.5 microM. Both glycine betaine-binding activity and affinity were not significantly modified whether or not the binding assays were done at high osmolarity. A 32 kDa osmotically inducible periplasmic protein, identified by SDS-PAGE, apparently corresponds to the glycine betaine-binding protein.