MTIP

MTIP (PF3D7_1246400) is a ~24 kDa myosin light chain that anchors the class XIV motor MyoA to the inner membrane complex (IMC), forming the core of the Plasmodium glideosome that drives gliding motility and host-cell invasion (PMID:12456714, PMID:16337961). It binds the 15-amino-acid C-terminal tail helix of MyoA with sub-micromolar affinity, and antibody-immobilized MTIP–MyoA complexes power actin filament gliding in vitro, establishing MTIP as the tail-anchoring component of a functional motor (PMID:12456714, PMID:16337961). Structural work resolved this interaction in atomic detail: MTIP undergoes large conformational changes in its C-terminal domain upon binding the MyoA tail, collapsing from extended into a pH-dependent compact state in which both domains wrap the helix, with MyoA His810 and Lys813 as decisive contacts (PMID:16547135, PMID:17628590); full-length PfMyoA structures further show MTIP is recognized by a degenerate IQ motif of the MyoA lever arm and contributes to the motor's mechano-chemical output (PMID:33046215). Within the reconstituted glideosome, MTIP binds the MyoA heavy chain independently, and is a prerequisite for subsequent binding of the essential light chain (ELC) to the adjacent neck region, the two light chains together enhancing motility beyond MTIP alone (PMID:28978649, PMID:28893907). MTIP function is regulated post-translationally: CDPK1 phosphorylates it at Ser-47 (PMID:18768477), phosphomimetic substitution at Ser-107/Ser-108 breaks the tight clamp around the MyoA tail (PMID:22932904), and a crosstalk between palmitoylation and phosphorylation modulates the MTIP–MyoA interaction and parasite invasion (PMID:36250062). In P. berghei motile stages MTIP is recruited apically independently of MyoA, indicating its IMC targeting does not require its motor partner (PMID:31874192).

1. 2003 High

   Establishing what tethers the invasion motor to the membrane, MTIP was defined as the MyoA-binding, IMC-localized protein that immobilizes the motor via the MyoA tail.

   Evidence Yeast two-hybrid with deletion mapping and immunofluorescence localization in sporozoites

   PMID:12456714

   Open questions at the time

   • Binding affinity and stoichiometry not yet quantified
   • Functional consequence of the tether for motility not shown
2. 2005 High

   To show the tether supports a working motor, the MTIP–MyoA complex was quantified and its motor activity measured, and a competing peptide was shown to disrupt the complex.

   Evidence Co-IP, fluorescence anisotropy binding, in vitro actin gliding assay, and peptide competition in parasite lysate

   PMID:16337961

   Open questions at the time

   • Predicted EF-hand role unresolved beyond calcium not modulating binding
   • Structural basis of binding not yet defined
3. 2006 High

   To explain how MTIP grips the MyoA tail, crystallography revealed large C-terminal-domain conformational changes forming hydrophobic pockets, and a MyoA helix peptide blocked parasite growth, validating the interface as a target.

   Evidence X-ray crystallography of MTIP and MTIP–MyoA tail complex plus growth inhibition assay

   PMID:16547135

   Open questions at the time

   • Physiological trigger selecting among the three observed conformations unknown
4. 2007 High

   To resolve which conformation operates in vivo, a pH-dependent compact complex enveloping the MyoA helix was defined, pinpointing MyoA His810 and Lys813 as governing contacts.

   Evidence X-ray crystallography with site-directed mutagenesis of MyoA residues

   PMID:17628590

   Open questions at the time

   • Whether pH-dependent switching occurs during invasion in the cell not demonstrated
5. 2008 High

   To identify regulation of the tether, MTIP was shown to be a CDPK1 substrate phosphorylated at Ser-47, linking a peripheral kinase to the glideosome.

   Evidence In vitro kinase assay with phosphosite mapping, phosphopeptide detection from lysates, immunofluorescence

   PMID:18768477

   Open questions at the time

   • Functional consequence of Ser-47 phosphorylation on MyoA binding not established
   • Stoichiometry of in vivo phosphorylation unclear
6. 2009 Medium

   Extending MTIP beyond the assembled motor, its association with Rab11A implicated an MTIP–myosin motor in vesicular trafficking for IMC maturation and cytokinesis.

   Evidence Co-IP of Rab11A with MTIP plus dominant-negative Rab11A and MyoA-tail mutant expression with microscopy

   PMID:19165333

   Open questions at the time

   • Functional link inferred from Rab11A manipulation rather than direct MTIP perturbation
   • Direct MTIP–Rab11A binding not mapped
7. 2010 Medium

   To confirm the crystallographic interface holds in solution, NMR identified the key binding determinants at the MTIP/MyoA interface.

   Evidence Solution protein NMR of the MTIP–MyoA complex

   PMID:20174678

   Open questions at the time

   • Dynamics of the conformational switch not quantified in this study
8. 2012 High

   To connect phosphorylation to motor regulation, Ser-107/Ser-108 were defined as a binding hotspot whose phosphomimetic substitution loosens the MTIP clamp on the MyoA tail.

   Evidence X-ray crystallography, NMR relaxation, biophysical binding assays, and S107E/S108E phosphomimetics tested against native MyoA in lysate

   PMID:22932904

   Open questions at the time

   • Kinase responsible for Ser-107/Ser-108 phosphorylation not identified
   • In vivo timing of this regulation during invasion unknown
9. 2013 Medium

   To assess the MyoA-binding groove as a drug/nanobody target, the MTIP-D3–nanobody structure showed nanobody binding occludes the groove via a two-helix shift.

   Evidence X-ray crystallography of the MTIP-D3–nanobody complex

   PMID:23831371

   Open questions at the time

   • Functional inhibition of the motor by the nanobody not demonstrated in parasites
10. 2013 Medium

    To generalize the pH-switch mechanism across species, the compact P. knowlesi MTIP–MyoA complex confirmed pH-dependent compact/extended states via His810-anchored hydrogen bonds.

    Evidence X-ray crystallography at multiple pH values in P. knowlesi

    PMID:23831369

    Open questions at the time

    • Physiological pH range experienced by the complex in vivo not defined
11. 2017 Medium

    To define the full light-chain composition of the glideosome, ELC was found to bind the MyoA neck adjacent to MTIP, and co-expression with both light chains enhanced motility beyond MTIP alone.

    Evidence Immunofluorescence across the P. berghei life cycle plus co-expression actin gliding assay

    PMID:28893907

    Open questions at the time

    • Regulatory interplay between MTIP and ELC during invasion not resolved
12. 2017 High

    To order light-chain assembly, reconstitution showed MTIP binds the MyoA heavy chain independently while ELC binding requires MTIP to be bound first.

    Evidence Recombinant reconstitution with systematic light-chain removal, actin gliding assay, and native co-purification

    PMID:28978649

    Open questions at the time

    • Structural basis for MTIP-dependent ELC recruitment not shown
    • Role of the MTIP N-terminal extension unresolved beyond no effect on velocity
13. 2018 Low

    Probing whether MTIP serves motors beyond MyoA, in vitro assays showed it can also interact with MyoB, hinting at a shared light-chain role.

    Evidence In vitro pull-down and MyoA-tail competition assays with recombinant proteins

    PMID:30094538

    Open questions at the time

    • In vitro pull-down only, not confirmed in vivo
    • Physiological relevance of MTIP–MyoB association unknown
14. 2019 Medium

    To test whether MTIP targeting depends on its motor partner, P. berghei genetics showed MTIP is recruited apically independently of MyoA and adopts a polarized distribution during gliding.

    Evidence Immunofluorescence of MyoA-knockout and HSP20-mutant P. berghei motile stages

    PMID:31874192

    Open questions at the time

    • Molecular determinant driving MTIP's MyoA-independent apical recruitment unknown
15. 2020 High

    To situate MTIP within the working motor, the full-length PfMyoA structure revealed MTIP recognition by a degenerate IQ motif of the lever arm and its contribution to motor mechano-chemistry.

    Evidence Full-length PfMyoA structure with kinetic ATPase and motility assays of MyoA variants

    PMID:33046215

    Open questions at the time

    • How phosphorylation-driven MTIP conformational changes alter the working stroke not directly measured
16. 2022 Medium

    To connect two PTMs to invasion, palmitoylation and phosphorylation of MTIP were shown to be mutually dependent, and disrupting this crosstalk abrogated MTIP–MyoA binding and synergistically inhibited invasion.

    Evidence Chemical inhibitor treatments, click-chemistry proteomics, interaction assay, and invasion checkerboard assay

    PMID:36250062

    Open questions at the time

    • Specific palmitoylation site(s) on MTIP not mapped
    • Enzymes mediating MTIP palmitoylation not identified
    • Effects inferred from inhibitors rather than site-directed mutagenesis

• How the multiple regulatory inputs on MTIP (Ser-47 by CDPK1, Ser-107/Ser-108, and palmitoylation crosstalk) are temporally coordinated to control motor engagement and disengagement during a single invasion cycle remains unresolved.
• No unified model linking each PTM to a defined step of gliding/invasion
• In vivo consequences of phosphosite mutations in living parasites not established