LMBD3

LMBD3 is a plasma membrane-embedded protein of Toxoplasma gondii that localizes to the apical annuli, structures where the parasite plasma membrane meets the inner membrane complex (IMC) (PMID:41256620). Its delivery to this compartment depends on the apical annuli protein AAP7: depletion of AAP7 prevents LMBD3 from reaching the annuli and instead causes it to accumulate among the secretory organelles, defining a novel AAP7-dependent trafficking route to the plasma membrane (PMID:41256620). At the annuli, LMBD3 is functionally connected to the IMC suture-embedded AAP proteins through AAP7, which appears to bridge plasma membrane-embedded LMBD3 to the IMC-embedded AAP scaffold (PMID:41256620). Beyond this localization and trafficking dependency, no molecular activity, substrate, or downstream function for LMBD3 has been characterized in the available corpus.

1. 2025 Medium

   Establishing where LMBD3 resides and how it gets there addressed whether it is an integral apical annuli component and revealed the trafficking machinery required for its delivery.

   Evidence Conditional depletion of AAP7 in ATc-resistant Toxoplasma gondii with fluorescence microscopy tracking LMBD3 localization (preprint)

   PMID:41256620

   Open questions at the time

   • The molecular function of LMBD3 at the apical annuli is unknown
   • Whether LMBD3 directly binds AAP7 or is trafficked via an indirect route was not resolved
   • Preprint, single lab, single localization method; not independently confirmed
2. 2025 Low

   Placing LMBD3 at the plasma membrane level of the annuli and linking it to IMC suture-embedded AAP proteins via AAP7 addressed how the two membrane systems at the annuli are physically bridged.

   Evidence Fluorescence microscopy and conditional protein depletion in ATc-resistant Toxoplasma gondii positioning LMBD3 relative to AAP proteins (preprint)

   PMID:41256620

   Open questions at the time

   • Plasma membrane embedding was inferred from localization, not confirmed biochemically
   • The direct interaction between LMBD3, AAP7, and the AAP scaffold has not been demonstrated biochemically
   • The functional consequence of the LMBD3-AAP7-AAP connection is uncharacterized

• The molecular activity of LMBD3 and the biological role of its recruitment to the apical annuli remain unknown.
• No enzymatic or transport activity assigned to LMBD3
• No phenotype for LMBD3 loss reported
• Mechanism of the AAP7-dependent trafficking route not defined