Recombinant Cronobacter sakazakii Prolipoprotein diacylglyceryl transferase (lgt)

Introduction to Lgt in Cronobacter sakazakii

Prolipoprotein diacylglyceryl transferase (Lgt) is an essential inner-membrane enzyme in Gram-negative bacteria that catalyzes the first step of lipoprotein maturation. In Cronobacter sakazakii, a pathogen linked to severe neonatal infections, Lgt modifies prolipoproteins by transferring a diacylglyceryl moiety from phosphatidylglycerol to a conserved cysteine residue in the lipobox sequence ([LVI][ASTVI][GAS]C) . This modification is critical for anchoring lipoproteins to the membrane and enabling downstream processing by LspA (signal peptidase II) and Lnt (N-acyl transferase) . Recombinant Lgt refers to the enzyme produced via heterologous expression systems for structural, functional, and inhibitor-screening studies.

Catalytic Mechanism

Lgt transfers the sn-1,2-diacylglyceryl group from phosphatidylglycerol to the thiol group of the conserved cysteine in prolipoproteins, forming a thioether bond . This step is indispensable for bacterial viability, as genetic depletion of lgt leads to outer membrane destabilization and cell death .

Topology and Conserved Motifs

Studies in Escherichia coli reveal that Lgt is anchored via seven transmembrane domains, with its N-terminus facing the periplasm and C-terminus oriented cytoplasmically . Key residues (e.g., Y26, N146, G154) in the Lgt signature motif (periplanar-facing) are essential for enzymatic activity . Homology modeling suggests C. sakazakii Lgt shares similar topology due to high sequence conservation (~50% identity with E. coli Lgt) .

Table 1: Comparative Features of Lgt Across Gram-Negative Bacteria

Inhibitor Development

Macrocyclic compounds (e.g., G2823, G2824) inhibit E. coli Lgt by binding its active site, leading to:

• Accumulation of unmodified prolipoproteins (e.g., pro-Lpp) .

• Outer membrane blebbing and increased antibiotic susceptibility .

• Bactericidal activity (MIC: 2–8 µg/mL against wild-type strains) .

These inhibitors do not exhibit cross-resistance with LspA or LolCDE inhibitors, highlighting target specificity .

Role in C. sakazakii Pathogenesis

Though direct evidence is sparse, Lgt’s role in lipoprotein maturation suggests it contributes to virulence by enabling:

• Membrane integrity: Critical for resisting serum complement and antibiotics .

• Virulence factor secretion: Lipoproteins mediate adhesion, nutrient acquisition, and immune evasion .

• Stress adaptation: C. sakazakii’s pan-genome includes genes for osmotic stress survival, potentially linked to lipoprotein function .

Table 2: Phenotypic Effects of Lgt Inhibition in Model Bacteria

Genomic and Evolutionary Insights

C. sakazakii’s accessory genome encodes niche-specific adaptations, including proton transport and antibiotic resistance genes (e.g., fos, mdf(A)) . While Lgt itself is part of the core genome, its conserved sequence suggests it is under strong purifying selection, making it a stable antibacterial target . Recombination events in lipoprotein-related genes (e.g., lpp) may influence resistance mechanisms .

Challenges and Future Directions

1. Structural studies: Cryo-EM or X-ray crystallography of recombinant C. sakazakii Lgt is needed to design species-specific inhibitors.

2. In vivo validation: Assess Lgt’s role in C. sakazakii infection models (e.g., neonatal meningitis).

3. Resistance profiling: Monitor for mutations in clinical isolates exposed to Lgt inhibitors.