The D-Alanyl carrier protein in Lactobacillus casei: cloning, sequencing, and expression of dltC

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Abstract
The incorporation of D-alanine into membrane-associated D-alanyl-lipoteichoic acid in Lactobacillus casei requires the 56-kDa D-alanine-D-alanyl carrier protein ligase (Dcl) and the 8.9-kDa D-alanyl carrier protein (Dcp). To identify and isolate the gene encoding Dcp, we have cloned and sequenced a 4.3-kb chromosomal fragment that contains dcl (dltA). In addition to this gene, the fragment contains three other genes, dltB, d1tC, and a partial dltD gene. dltC (246 nucleotides) was subcloned from this region and expressed in Escherichia coli. The product was identified as apo-Dcp lacking the N-terminal methionine (8,787.9 Da). The in vitro conversion of the recombinant apo-Dcp to holo-Dcp by recombinant E. coli holo-ACP synthase provided Dcp which accepts activated D-alanine in the reaction catalyzed by Bcl. The recombinant D-alanyl-Dcp was functionally identical to native D-alanyl-Dcp in the incorporation of D-alanine into lipoteichoic acid. L. casei Dcp is 46% identical to the putative product of dltC in the Bacillus subtilis dlt operon (M. Perego, P. Glaser, A. Minutello, M. A. Strauch, K. Leopold, and W. Fischer, J. Biol. Chem. 270:15598-15606, 1995), and therefore, this gene also encodes Dcp. Comparisons of the primary sequences and predicted secondary structures of the L. casei and B. subtilis Dcps with that of the E. coli acyl carrier protein (ACP) were undertaken together with homology modeling to identify the functional determinants of the donor and acceptor specificities of Dcp. In the region of the phospho-pantetheine attachment site, significant similarity between Dcps and ACPs was observed. This similarity may account for the relaxed acceptor specificity of the Dcps and ACPs in the ligation Of D-alanine catalyzed by Dcl. In contrast, two Dcp consensus sequences, KXXVLDXLA and DXVKXNXD, share little identity with the rest of the ACP family and, thus, may determine the donor specificity of D-alanyl-Dcp in the D-alanylation of membrane-associated D-alanyl-lipoteichoic acid.