Do Sequence Repeats Play an Equivalent Role in the Choline-binding Module of Pneumococcal LytA Amidase?

Abstract LytA amidase breaks down theN-acetylmuramoyl-l-alanine bonds in the peptidoglycan backbone of Streptococcus pneumoniae. Its polypeptide chain has two modules: the NH2-terminal module, responsible for the catalytic activity, and the COOH-terminal module, constructed by six tandem repeats of 20 or 21 amino acids (p1–p6) and a short COOH-terminal tail. The polypeptide chain must contain at least four repeats to efficiently anchor the autolysin to the choline residues of the cell wall. Nevertheless, the catalytic efficiency decreases by 90% upon deletion of the final tail. The structural implications of deleting step by step the two last (p5 and p6) repeats and the final COOH-tail and their effects on choline-amidase interactions have been examined by comparing four truncated mutants with LytA amidase by means of different techniques. Removal of this region has minor effects on secondary structure content but significantly affects the stability of native conformations. The last 11 amino acids and the p5repeat stabilize the COOH-terminal module; each increases the module transition temperature by about 6 °C. Moreover, the p5motif also seems to participate, in a choline-dependent way, in the stabilization of the NH2-terminal module. The effects of choline binding on the thermal stability profile of the mutant lacking the p5 repeat might reflect a cooperative pathway providing molecular communication between the choline-binding module and the NH2-terminal region. The three sequence motives favor the choline-amidase interaction, but the tail is an essential factor in the monomer ↔ dimer self-association equilibrium of LytA and its regulation by choline. The final tail is required for preferential interaction of choline with LytA dimers and for the existence of different sets of choline-binding sites. Thep6 repeat scarcely affects the amidase stability but could provide the proper three-dimensional orientation of the final tail.