SDHA

In Toxoplasma gondii, SDHA (TgSdhA) is the catalytic core subunit of succinate dehydrogenase (complex II) of the mitochondrial electron transport chain, together with TgSdhB, and localizes to the parasite mitochondrion (PMID:38835243, PMID:33651838, PMID:40050648). Membrane anchoring of the TgSdhA/TgSdhB catalytic core and full SDH enzymatic activity depend on the myzozoan-specific membrane subunit TgMPODD, whose loss abolishes SDH activity (PMID:38835243). Through this SDH function, TgSdhA supports mitochondrial respiration and the TCA cycle during the lytic cycle, and its disruption impairs respiration, reduces asexual proliferation, and triggers spontaneous differentiation into bradyzoites (PMID:38079448, PMID:40050648). SDHA is important for optimal proliferation and for full virulence in mice, yet is dispensable for basic parasite survival (PMID:38079448, PMID:40050648).

1. 2021 Medium

   Establishing where SDHA sits in the parasite's respiratory proteome was the first step; complexome profiling assigned it to complex II, placing SDHA within the Toxoplasma mitochondrial electron transport chain.

   Evidence Complexome profiling (size-exclusion chromatography coupled to quantitative mass spectrometry)

   PMID:33651838

   Open questions at the time

   • No direct functional validation of SDHA-specific enzymatic contribution
   • Membrane-anchoring architecture unresolved
2. 2023 High

   Whether SDHA-containing SDH is functionally required was answered by showing it is essential for SDH assembly and activity and links respiration to developmental fate, since its disruption impaired respiration and triggered bradyzoite differentiation.

   Evidence Conditional knockdown/genetic disruption of SDH subunits with respiration, growth, assembly, and differentiation readouts

   PMID:38079448

   Open questions at the time

   • Mechanism linking respiratory failure to differentiation not defined
   • Membrane subunit requirements not yet identified
3. 2024 High

   How the catalytic core attaches to the divergent membrane component was resolved by identifying the myzozoan-specific subunit TgMPODD as the anchor for TgSdhA/TgSdhB, defining the assembly logic of the parasite's atypical complex II.

   Evidence Genetic tagging, co-immunoprecipitation/complexome analysis, conditional knockdown, SDH activity and O2 consumption assays

   PMID:38835243

   Open questions at the time

   • Structure of the assembled myzozoan SDH not determined
   • Roles of other membrane subunits beyond TgMPODD not fully dissected
4. 2025 Medium

   The physiological consequence of losing SDHA in vivo was defined: clean deletion renders parasites nearly avirulent and reduces asexual reproduction while remaining dispensable for survival, establishing SDHA as a virulence/proliferation determinant rather than an absolute essential gene.

   Evidence Genetic knockout, mouse virulence model, mitochondrial localization, metabolic phenotyping

   PMID:40050648

   Open questions at the time

   • Single-lab, single-study in vivo phenotype
   • Metabolic rewiring enabling survival without SDHA not defined

• The molecular basis by which respiratory/SDH failure drives spontaneous bradyzoite differentiation, and the compensatory metabolism permitting survival despite SDHA loss, remain unresolved.
• Signal coupling respiration to differentiation unknown
• No structural model of the myzozoan SDH complex
• Metabolic bypass supporting viability uncharacterized