PEX5

Toxoplasma gondii Pex5 is a functional peroxisomal targeting signal 1 (PTS1) import receptor that mediates cargo delivery into peroxisome-like organelles in the parasite (PMID:30158461). TgPex5 expressed in Pex5-deleted Saccharomyces cerevisiae rescues the PTS1 import defect, establishing that it retains bona fide receptor activity (PMID:30158461), and its PTS1-binding domain physically engages a PTS1 ligand derived from the parasite sterol carrier protein SCP2, defining a direct receptor-cargo interaction (PMID:30158461). Consistent with an active receptor, GFP bearing an enhanced PTS1 signal localizes to peroxisome-like bodies when expressed in T. gondii, demonstrating a functional PTS1-dependent import pathway in the parasite (PMID:30158461). Beyond cargo recognition, the mechanism of membrane translocation downstream of TgPex5 has not been characterized in the available corpus.

1. 2018 Medium

   It was unknown whether Toxoplasma encoded a functional PTS1 import receptor; heterologous complementation established TgPex5 as a bona fide receptor rather than a mere sequence homolog.

   Evidence Complementation of Pex5-deleted S. cerevisiae by TgPex5, restoring PTS1 protein import

   PMID:30158461

   Open questions at the time

   • Receptor activity shown only in a heterologous yeast system, not by direct loss-of-function in the parasite
   • Cargo repertoire imported by TgPex5 in vivo not defined
2. 2018 Medium

   To connect receptor activity to a physiological cargo, the PTS1-binding domain of TgPex5 was shown to bind a PTS1 ligand from the parasite SCP2, providing a direct receptor-cargo interaction.

   Evidence In vitro binding assay with recombinant TgPex5 PTS1-binding domain and an SCP2-derived PTS1 peptide

   PMID:30158461

   Open questions at the time

   • Binding shown with a peptide ligand rather than full-length native SCP2
   • Peroxisomal localization of SCP2 in vivo not demonstrated
   • Affinity and specificity relative to other PTS1 cargo not established
3. 2018 Medium

   To show the import pathway is operational in the organism itself, a reporter carrying an enhanced PTS1 signal was tracked in the parasite, revealing PTS1-dependent targeting to peroxisome-like bodies.

   Evidence Fluorescence microscopy of GFP-ePTS1 expressed in T. gondii

   PMID:30158461

   Open questions at the time

   • Does not directly demonstrate TgPex5 dependence of the observed targeting
   • Nature and biogenesis of the peroxisome-like bodies not resolved
4. 2018 Low

   Comparative genomics addressed how import proceeds after cargo capture, finding that the canonical docking partners Pex13 and Pex14 are absent from coccidian genomes, implying a nonconventional translocation mechanism downstream of Pex5.

   Evidence Comparative genomic survey of apicomplexan genomes

   PMID:30158461

   Open questions at the time

   • Absence is inferred bioinformatically without experimental validation of Pex13/Pex14 function loss
   • The alternative docking/translocation machinery is unidentified
5. 2018 Low

   Broader phylogenetic analysis used Pex5 as a peroxisome marker to argue that apicomplexans possess peroxisomes, contradicting prior claims that these organelles are absent.

   Evidence Comparative genomics and phylogenetic analysis across alveolate species

   PMID:29202176

   Open questions at the time

   • Inference from marker presence rather than direct functional characterization of TgPex5
   • Does not establish the full peroxisomal proteome or organelle physiology in T. gondii

• The membrane translocation machinery operating downstream of TgPex5 in the absence of Pex13/Pex14, and the in vivo essentiality and full cargo repertoire of TgPex5, remain unknown.
• No identification of the nonconventional docking/translocation factors
• No direct parasite loss-of-function characterization of TgPex5
• Full set of PTS1 cargo proteins undefined