LipB

LipB (PF3D7_0823600) is an apicoplast-localized octanoyl-ACP:protein N-octanoyltransferase that catalyzes the first committed step of de novo lipoic acid synthesis, attaching the lipoyl/octanoyl moiety to apicoplast target proteins including the apicoplast pyruvate dehydrogenase complex (PMID:15225307, PMID:18069893). Its apicoplast targeting was established by GFP-fusion imaging of its N-terminal targeting sequence, and its enzymatic activity was confirmed by complementation of E. coli lipB-deficient mutants, placing it in the apicoplast de novo lipoic acid synthesis pathway distinct from the mitochondrial salvage route (PMID:15225307). In P. falciparum intraerythrocytic stages LipB is dispensable: its disruption abolishes >90% of apicoplast lipoic acid without impairing asexual growth, with residual lipoylation attributed to the dual-targeted ligase LplA2, indicating functional redundancy in apicoplast lipoylation (PMID:18069893). LipB becomes functionally important in other life stages and under metabolic stress: in P. berghei it is required for late liver-stage maturation and sporozoite infectivity, and blood-stage growth of LipB-deficient parasites is reduced under lipid-restricted conditions (PMID:23490300); in P. falciparum, LipB deletion alters apicoplast and cytosolic redox regulator expression and reshapes central carbon metabolism (glycolysis and TCA cycle), and prevents completion of sporogony in Anopheles, abolishing infectious sporozoite production (PMID:33713652).

1. 2004 High

   Establishing where LipB acts and whether it is a functional lipoic acid synthesis enzyme was the first step in defining a distinct apicoplast lipoylation pathway in Plasmodium.

   Evidence GFP-fusion live imaging of the N-terminal targeting sequence plus bacterial complementation of lipB-deficient E. coli in P. falciparum

   PMID:15225307

   Open questions at the time

   • Enzymatic mechanism and endogenous substrate spectrum not directly resolved in the parasite
   • Overexpressed targeting-sequence fusion rather than endogenous protein localization
2. 2007 High

   Whether LipB is essential for blood-stage parasites was resolved, revealing that de novo apicoplast lipoylation is buffered by redundancy despite LipB supplying most apicoplast lipoic acid.

   Evidence Gene disruption in P. falciparum with lipoic acid quantification and lipoylation-status assays

   PMID:18069893

   Open questions at the time

   • Molecular basis and regulation of LplA2 compensation not defined
   • Does not address other life stages
3. 2013 High

   The stage-specific requirement for LipB was defined, showing it is dispensable in blood stages but essential for liver-stage maturation and sporozoite infectivity, and conditionally important under lipid restriction.

   Evidence P. berghei knockout with mouse infection assays, in vitro liver-stage development, and growth under lipid-restricted conditions (8-bromo-octanoate, clofibrate)

   PMID:23490300

   Open questions at the time

   • Which liver-stage substrates depend on LipB not identified
   • Mechanistic link between lipoylation loss and late liver-stage arrest not established
4. 2021 Medium

   The downstream metabolic and transmission consequences of losing LipB were mapped, connecting its loss to altered redox regulation, central carbon metabolism, and a block in sporogony.

   Evidence P. falciparum 3D7/NF54 knockout with metabolomics, redox regulator expression profiling, and mosquito infection assays

   PMID:33713652

   Open questions at the time

   • Causal chain from lipoylation loss to redox and carbon-metabolism changes not resolved
   • Single lab; mechanism of sporogony arrest not defined

• How LipB and the redundant ligase LplA2 partition apicoplast lipoylation, and the direct enzyme-substrate mechanism by which LipB modifies its targets in the parasite, remain unresolved.
• No structural model of LipB in Plasmodium
• Direct in-parasite substrate profiling absent
• Mechanism of LplA2 functional overlap unknown