Biosynthesis of lipoteichoic acid in Lactobacillus rhamnosus:: Role of DltD in D-alanylation

The dlt operon (dltA to dltD) of Lactobacillus rhamnosus 7469 encodes four proteins responsible for the esterification of lipoteichoic acid (LTA) by D-alanine. These esters play an important role in controlling the net anionic charge of the poly (GroP) moiety of LTA, dltA and dltC encode the D-alanine-D-alanyl carrier protein ligase (Dcl) and D-alanyl carrier protein (Dcp), respectively. Whereas the functions of DltA and DltC are defined, the functions of DltB and DltD are unknown. To define the role of DltD, the gene was cloned and sequenced and a mutant was constructed by insertional mutagenesis of dltD from Lactobacillus casei 102S. Permeabilized cells of a dltD::erm mutant lacked the ability to incorporate D-alanine into LTA, This defect was complemented by the expression of DltD from pNZ123/dlt. In in vitro assays, DltD bound Dcp for ligation with D-alanine by Dcl in the presence of ATP, In contrast, the homologue of Dcp, the Escherichia coli acyl carrier protein (ACP), involved in fatty acid biosynthesis, was not bound to DltD and thus was not ligated,vith D-alanine. DltD also catalyzed the hydrolysis of the mischarged D-alanyl-ACP, The hydrophobic N-terminal sequence of DltD was required for anchoring the protein in the membrane. It is hypothesized that this membrane-associated DltD facilitates the binding of Dcp and Dcl for ligation of Dcp with D-alanine and that the resulting D-alanyl-Dcp is translocated to the primary site of D-alanylation.