SLF

SLF (signalling linking factor) is a component of the guanylate cyclase (GC) signalling complex in Toxoplasma gondii that couples cGMP-dependent signalling to the early induction of egress and the invasion machinery (PMID:35538310, PMID:36200777). In tachyzoites, SLF functions within the GC complex alongside GC, CDC50.1, and UGO, and its conditional depletion impairs microneme exocytosis, conoid extrusion, and gliding motility, thereby blocking both invasion and egress (PMID:36200777). SLF activity is also required across the parasite life cycle: SLF-deficient bradyzoites produce reduced cyst burdens and fail to establish infection upon oral challenge, implicating SLF in the natural transmission route (PMID:36200777). Beyond its placement within the GC complex and these loss-of-function phenotypes, the biochemical basis of SLF's contribution to the complex has not been characterized in the available corpus.

1. 2022 Medium

   Establishing which factors control the timing of egress, SLF was identified as an integral part of a signalling complex required for the early induction of egress, defining a discrete step where its loss arrests the process.

   Evidence SplitCas9 phenotypic screen and DiCre conditional knockout with F-actin/apicoplast indicator strain in T. gondii

   PMID:35538310

   Open questions at the time

   • The molecular activity of SLF and its direct binding partners were not defined
   • The specific early egress signalling step blocked was inferred from indicator phenotypes rather than direct biochemistry
2. 2022 High

   To place SLF mechanistically, conditional depletion showed it acts within the cGMP-dependent GC complex, and that its loss simultaneously disrupts microneme exocytosis, conoid extrusion, and motility — linking a single factor to the coordinated invasion/egress program.

   Evidence Conditional depletion via stage-specific promoter swap with microneme exocytosis, conoid extrusion, motility, invasion and egress assays in tachyzoites

   PMID:36200777

   Open questions at the time

   • How SLF physically integrates with GC, CDC50.1, and UGO was not resolved
   • Whether SLF has enzymatic or purely structural/scaffolding function is unknown
3. 2022 Medium

   To test relevance beyond the acute stage, SLF-deficient bradyzoites were shown to be defective in cyst formation and unable to establish oral infection, establishing SLF as essential for the natural transmission route and chronic infection.

   Evidence Stage-specific promoter swap generating SLF-deficient bradyzoites with oral infection mouse model and cyst burden quantification

   PMID:36200777

   Open questions at the time

   • The bradyzoite-stage molecular events downstream of SLF were not dissected
   • Whether the transmission defect reflects the same microneme/motility functions seen in tachyzoites is untested

• The biochemical function of SLF within the GC complex — its direct interactions, structural role, and how it links cGMP signalling to microneme discharge and conoid extrusion — remains undefined.
• No structure or direct interaction map for SLF within the GC complex
• No defined molecular/enzymatic activity for SLF
• Mechanism by which SLF couples cGMP signalling to the invasion/egress motor is unknown