Inter-chain transacylation of D-alanine ester residues of lipoteichoic acid: a unique mechanism of membrane communication.

Lipoteichoic acid is an important example of an amphiphile that appears to play a vital role in bacterial growth. A number of functions have been ascribed to this polymer. These include the regulation of autolytic enzymes, the binding of Mgz+ for enzyme function, and the assembly of wall polymers. One of the key components which has been proposed to modulate these functions is the substituent D-alanine. James Baddiley and his co-workers discovered that this amino acid is an important component of most teichoic acids (Armstrong et al., 1959). Thus, esterification of teichoic acids by D-alanine may be an important step in controlling the function of this polymer in wall assembly. In Lactobacillus casei the incorporation of D-alanine into lipoteichoic acid is accomplished in a two-step reaction sequence. The first step of this sequence was discovered in 1960 when Baddiley & Neuhaus showed that a variety of Gram-positive organisms have the ability to activate Dalanine. The second step of this sequence was not detected until Reusch & Neuhaus (1971) recognized that the incorporation of D-alanine into teichoic acids in vitro required the endogenous membrane teichoic acid to act as acceptor. All attempts to demonstrate the incorporation of D-alanine into isolated wall teichoic acid proved to be elusive and it was reasoned that the teichoic acid has to be intimately associated with the membrane in a particular conformation or environment (Neuhaus et aZ., 1974). On the basis of this hypothesis a system for the incorporation of D-alanine into membranes was found that required both a supernatant fraction and ATP, and was stimulated by metal ions. Linzer & Neuhaus (1973) showed that the supernatant fraction could be replaced by the D-alanine-activating enzyme and a protein factor designated D-alanine: membrane acceptor ligase. L. casei was chosen for these experiments because, firstly, it has a high concentration of the