TGME49_214090

In Toxoplasma gondii, signal peptidase initiates a two-step proteolytic maturation pathway for micronemal preproproteins, cleaving the signal peptide to convert preproproteins such as TgMIC5 into proproteins that are subsequently processed to their mature secreted forms stored in the micronemes (PMID:11087916). The signal peptidase cleavage event defines the N-terminus of the MIC propeptides, and the aliphatic residues exposed immediately after the cleavage site—valine at +3 of proM2AP and leucine at +1 of proMIC5—are required for forward trafficking of these proteins to the micronemes (PMID:21438967). Beyond its role in generating trafficking-competent propeptide N-termini, no further mechanistic detail of the signal peptidase itself has been characterized in the available corpus.

1. 2000 Medium

   Established that a micronemal protein is not synthesized in mature form but passes through a signal-peptidase-dependent maturation cascade, defining the proteolytic logic of microneme protein biogenesis.

   Evidence Molecular cloning, cell fractionation, immunofluorescence, and immunoelectron microscopy of TgMIC5 in T. gondii

   PMID:11087916

   Open questions at the time

   • Does not identify the signal peptidase enzyme itself or its catalytic subunits
   • The identity of the protease performing the second maturation step is not established
   • Functional consequence of blocking signal peptidase cleavage is not tested
2. 2011 High

   Resolved why signal peptidase cleavage matters beyond signal-peptide removal by showing that the aliphatic residue it exposes at the propeptide N-terminus is the trafficking determinant directing proproteins to the micronemes.

   Evidence N-terminal deletion, site-directed mutagenesis, and propeptide domain-swapping trafficking assays across multiple MICs and an Eimeria ortholog in T. gondii

   PMID:21438967

   Open questions at the time

   • The receptor or machinery that reads the aliphatic N-terminal residue is not identified
   • How the exposed residue is recognized during forward trafficking is unresolved
   • The signal peptidase enzyme is inferred as the cleaving activity but not directly characterized

• The molecular identity, subunit composition, and catalytic mechanism of the Toxoplasma signal peptidase itself remain uncharacterized in this corpus.
• No direct biochemical isolation or structural characterization of the signal peptidase enzyme
• No identification of the downstream protease mediating the second maturation cleavage
• No identification of the trafficking receptor recognizing the aliphatic N-terminal residue