TGME49_280740

In Toxoplasma gondii, signal peptidase carries out the first proteolytic step in the maturation of secretory microneme proteins, cleaving preproproteins such as TgMIC5 to generate a proprotein intermediate that is subsequently processed to the mature, micronemal form (PMID:11087916). This cleavage defines the N-terminus of the resulting MIC propeptide, and the aliphatic residue positioned at or near the cleavage site (valine at +3 of proM2AP, leucine at +1 of proMIC5) is required for forward trafficking of the proprotein to the micronemes; conservative valine-to-leucine substitution preserves targeting, establishing a shared aliphatic side-chain-dependent trafficking mechanism downstream of signal peptidase cleavage (PMID:21438967). Beyond its role in defining these propeptide N-termini and the downstream trafficking determinant, no further mechanistic detail on the signal peptidase enzyme itself has been characterized in the available corpus.

1. 2000 Medium

   Establishing that TgMIC5 is a preproprotein clarified that microneme proteins undergo an ordered maturation cascade, with signal peptidase cleavage as the defined first proteolytic step preceding storage and secretion.

   Evidence Molecular cloning, cell fractionation, secretion assays, immunofluorescence and immuno-EM in Toxoplasma gondii

   PMID:11087916

   Open questions at the time

   • The identity and biochemistry of the signal peptidase enzyme itself were not characterized
   • The nature of the further processing step yielding the mature 22 kDa product was not defined
2. 2011 High

   Mutagenesis defined why signal peptidase cleavage matters for trafficking: the aliphatic residue it exposes at the propeptide N-terminus is the determinant required for forward targeting of MIC proproteins to micronemes.

   Evidence N-terminal deletion and site-directed mutagenesis of propeptides with subcellular localization readouts and propeptide swapping across T. gondii MICs and EtMIC5

   PMID:21438967

   Open questions at the time

   • The trafficking machinery that reads the aliphatic N-terminal residue was not identified
   • The enzyme mediating the subsequent maturation of proprotein to mature MIC was not established
   • The generality beyond the tested MIC substrates was not fully explored

• The molecular identity, structure, and catalytic mechanism of the Toxoplasma signal peptidase, and how the exposed aliphatic N-terminus is recognized by the trafficking machinery, remain unresolved in the available corpus.
• No structural or enzymatic characterization of the signal peptidase itself
• No receptor or adaptor identified for the aliphatic N-terminal trafficking signal