Subsystem: Polyglycerolphosphate lipoteichoic acid biosynthesis

This subsystem's description is:

Enzymes involved in the synthesis of glycolipid anchor and lipoteichoic acid (LTA) backbone, described in Listeria monocytogenes, and extended to other organisms using a homologous system for LTA backbone biosynthesis.

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Enzymes involved in the synthesis of glycolipid anchor and lipoteichoic acid (LTA) backbone, described in Listeria monocytogenes, and extended to other organisms using a homologous system for LTA backbone biosynthesis.
A typical Gram-positive envelope is composed of peptidoglycan, proteins, often capsular polysaccharides and secondary wall polymers, which include wall teichoic acid (WTA), a polymer covalently linked to peptidoglycan, and lipoteichoic acid (LTA), a polymer tethered by a lipid anchor to the bacterial membrane. LTAs are polymers of glycerophosphate substituted with Ala, galactose, and lipid residues. The glycerophosphate chain of LTAs is uncovalently anchored to the outer leaflet of the plasma membrane through a lipid anchor, composed of galactose bound to glycerol and substituted with fatty acids.

The structure of LTA varies among organisms. In L. monocytogenes, the polyglycerolphosphate LTA backbone is substituted with both d-alanines and α-galactosyl residues and linked to the bacterial membrane via glycolipids Gal(α1-2)Glc(α1-3)-diacylglycerol (Gal-Glc-DAG) or Gal(α1-2)Ptd-6Glc(α1-3)DAG (Gal-Ptd-6Glc-DAG), in which the glucose moiety is lipidated at position 6 with a phosphatidyl (Ptd) group.

LTA backbone biosynthesis: L. monocytogenes uses a two-enzyme system for LTA synthesis (see Illustration). LtaP acts as an LTA primase and produces GroP-glycolipids and LtaS functions as LTA synthase and generates the polyglycerolphophate backbone. The two enzymes are paralogous.
In contrast to L. monocytogenes, S. aureus apparently synthesizes LTA with a single enzyme, while members of the Bacilli possess several paralogs. The biological significance why some bacteria use a one-enzyme and other bacteria use a multienzyme system for LTA synthesis is not known.
While it is not clear why different Gram-positive bacteria use one or multiple enzymes for LTA synthesis, in general there seems to be a correlation between the number of genome-encoded LtaS-like proteins and bacterial shape. Coccoid Staphylococcus spp., Streptococcus spp. (with exception of S. pneumoniae, which does not produce a polyglycerolphosphate-type LTA and does not encode an LtaS-like protein) and Lactococcus lactis strains encode one LtaS protein; ellipsoid-shaped E. faecalis strains, rod-shaped Listeria spp. and with a few exceptions rod-shaped Lactobacillus spp. encode two proteins; and the majority of Bacillus spp., rod-shaped bacteria with a more complex developmental cycle, encode multiple LtaS-like proteins, with at least partially overlapping functions.

The multiple paralogous LtaS are organized in this SS according to the phylogenetic clade to which they belong.

Although Clostridia do not possess typical LTA backbones, there are homologs to LtaS, but they group into a distinct clade with 2 subfamilies (Types IVa and IVb).

Glycolipid anchor biosynthesis: Genes in the laf operon are responsible for glycolipid anchor biosynthesis. LafA and LafB are necessary for the production of Glc-DAG and Gal-Glc-DAG respectively. These enzymes likely use the nucleotide-activated sugars UDP-glucose and UDP-galactose as substrates. A third gene, lafC, predicted to code for an integral membrane protein with eight transmembrane helices is also part of the operon. LafC plays an accessory function in glycolipid and LTA synthesis as inactivation of LafC results in minor changes in the glycolipid profile.

Legend to Accompanying Illustration: Model for glycolipid and LTA synthesis in L. monocytogenes.

The cytoplasmic glycosyltransferases Lmo2555 (LafA, LTA anchor formation protein A; shown in blue) and Lmo2554 (LafB; shown in red) synthesize Glc-DAG and Gal-Glc-DAG, respectively, presumably using nucleotide-activated sugars UDP-Glc and UDP-Gal as substrates. Lmo2553 (LafC, shown in grey) is a membrane protein of unknown function and likely acts downstream of LafA and LafB in the glycolipid synthesis pathway. L. monocytogenes uses a two-enzyme system for the subsequent polyglycerolphosphate LTA chain formation. The LTA primase Lmo0644 (LtaP, shown in light orange) transfers the initial glycerolphosphate (black circle) derived from phosphatidylglycerol (PG) onto Gal-Glc-DAG, resulting in the production of GroP-Gal-Glc-DAG. The LTA synthase Lmo0927 (LtaS, shown in orange) then transfers additional glycerolphosphate residues onto GroP-Gal-Glc-DAG, thereby forming the polyglycerolphosphate backbone chain of LTA.