Characterization of two Lactococcus lactis zinc membrane proteins, Llmg_0524 and Llmg_0526, and role of Llmg_0524 in cell wall integrity

Background: Due to its extraordinary chemical properties, the cysteine amino acid residue is often involved in protein folding, electron driving, sensing stress, and binding metals such as iron or zinc. Lactococcus lactis, a Gram-positive bacterium, houses around one hundred cysteine-rich proteins (with the CX2C motif) in the cytoplasm, but only a few in the membrane. Results: In order to understand the role played by this motif we focused our work on two membrane proteins of unknown function: Llmg_0524 and Llmg_0526. Each of these proteins has two CX2C motifs separated by ten aminoacid residues (CX2CX10CX2C). Together with a short intervening gene (llmg_0525), the genes of these two proteins form an operon, which is induced only during the early log growth phase. In both proteins, we found that the CX2CX10CX2C motif chelated a zinc ion via its cysteine residues, but the sphere of coordination was remarkably different in each case. In the case of Llmg_0524, two of the four cysteines were ligands of a zinc ion whereas in Llmg_0526, all four residues were involved in binding zinc. In both proteins, the cysteine-zinc complex was very stable at 37 degrees C or in the presence of oxidative agents, suggesting a probable role in protein stability. We found that the complete deletion of llmg_0524 increased the sensitivity of the mutant to cumene hydroperoxide whereas the deletion of the cysteine motif in Llmg_0524 resulted in a growth defect. The latter mutant was much more resistant to lysozyme than other strains. Conclusions: Our data suggest that the CX2CX10CX2C motif is used to chelate a zinc ion but we cannot predict the number of cysteine residue involved as ligand of metal. Although no other motif is present in sequence to identify roles played by these proteins, our results indicate that Llmg_0524 contributes to the cell wall integrity.