The contribution of the cell wall to a transmembrane calcium gradient could play a key role in Bacillus subtilis protein secretion

A weak Ca²⁺-binding site (K_a= 0.8× 10³ M^?1, at pH7) was identified in the mature part of levansucrase. An amino acid substitution (Thr-236 →lle) in this site alters simultaneously the affinity for calcium, the folding transition and the efficiency of the secretion process of levansucrase. Moreover, the ability of the Bacillus subtilis cell wall to concentrate calcium ions present in the culture medium was studied. We confirm the results of Beveridge and Murray who showed that the concentration factor is about 100 to 120 times. This property preserves a high concentration of Ca²⁺ (>2 mM) on the external side of the cytoplasmic membrane, even in the absence of further Ca2+ supplementation in the growth medium. Such local conditions allow the spontaneous unfolding folding transition of levansucrase en route for secretion. Since several exocellular proteins of B. subtilis are calcium-binding proteins, we propose that the high concentration of calcium ion in the microenvironment of the cell wall may play a key role in the ultimate step of their secretion process.