Carboxy-terminal deletion analysis of the major pneumococcal autolysin

Autolysins are endogenous enzymes that specifically degrade the covalent bonds of the cell walls and eventually can induce bacterial lysis. One of the best-characterized autolysins, the major pneumococcal LytA amidase, has evolved by the fusion of two domains, the N-terminal catalytic domain and the C-terminal domain responsible for the binding to cell walls. The precise biochemical role played by the six repeat units that form the C-terminal domain of the LytA amidase has been investigated by producing serial deletions. Biochemical analyses of the truncated mutants revealed that the LytA amidase must contain at least four units to efficiently recognize the choline residues of pneumococcal cell walls. The loss of an additional unit dramatically reduces its hydrolytic activity as well as the binding affinity, suggesting that the catalytic efficiency of this enzyme can be considerably improved by keeping the protein attached to the cell wall substrate. Truncated proteins lacking one or two repeat units were more sensitive to the inhibition by free choline than the wild-type enzyme, whereas the N-terminal catalytic domain was insensitive to this inhibition. In addition, the truncated proteins were inhibited by deoxycholate (DOC), and the expression of a LytA amidase lacking the last 11 amino acids in Streptococcus pneumoniae M31, a strain having a deletion in the lytA gene, conferred to the cells an atypical phenotype (Lyt+ DOC-) (cells autolysed at the end of the stationary phase but were not sensitive to lysis induced by DOC), which has been previously observed in some clinical isolates of pneumococci. Our results are in agreement with the existence of several choline-binding sites and suggest that the stepwise acquisition of the repeat units and the tail could be considered an evolutionary advantage for the enzyme, since the presence of these motifs increases its hydrolytic activity.