ELC2

ELC2 (essential light chain 2) is a component of the MyoA glideosome in Toxoplasma gondii, binding the MyoA lever arm alongside the light chains ELC1 and MLC1 (PMID:26374117). ELC1 and ELC2 are functionally redundant, but the presence of at least one is required for efficient MyoA-driven gliding motility, host cell invasion, and egress (PMID:26374117). Within the lever arm, Ca2+ promotes intermolecular contacts between ELC1/ELC2 and the adjacent MLC1, stabilizing this domain; point mutations that ablate either the Ca2+ interaction or the inter-light-chain interface degrade the quality, displacement, and speed of gliding motility (PMID:26374117). Beyond this Ca2+-dependent lever-arm stabilization role, no further mechanistic detail for ELC2 has been characterized in the available corpus.

1. 2015 Medium

   Established that the MyoA glideosome contains a previously unrecognized essential light chain, ELC2, and defined its place in the motor complex — answering whether MyoA lever-arm composition extends beyond ELC1 and MLC1.

   Evidence Genetic loss-of-function characterization with invasion, egress, and gliding-motility readouts identifying ELC2 as a glideosome component in Toxoplasma gondii

   PMID:26374117

   Open questions at the time

   • The molecular basis of ELC1/ELC2 redundancy (identical vs. distinct contributions) is not resolved
   • No structural model of ELC2 bound to the MyoA lever arm at atomic resolution
   • Regulation of ELC2 expression or assembly timing is uncharacterized
2. 2015 Medium

   Defined how Ca2+ mechanically stabilizes the MyoA lever arm — showing that ELC1/ELC2 contacts with MLC1 are Ca2+-dependent and functionally required for productive motility.

   Evidence Molecular dynamics simulations combined with point mutagenesis of the Ca2+ and inter-light-chain interfaces and gliding motility assays in Toxoplasma gondii

   PMID:26374117

   Open questions at the time

   • The physiological Ca2+ signal that triggers lever-arm stabilization in vivo is not defined
   • Quantitative force/step-size consequences of destabilization at the single-motor level are not measured
   • Whether ELC1 and ELC2 differ in Ca2+ sensitivity is not addressed

• It remains unknown whether ELC1 and ELC2 have any non-redundant functions, how ELC2 assembly is regulated, and what its structure in the assembled glideosome is.
• No distinction of ELC1 vs. ELC2 specific roles
• No atomic-resolution structure of the ELC2-containing lever arm
• No characterization of ELC2 outside the gliding-motility context