PRF

TgPRF is a Toxoplasma gondii profilin that combines a cytoskeletal actin-regulatory function with a potent role as a pathogen-associated molecular pattern driving innate immune activation (PMID:20851125). Its crystal structure reveals a parasite-specific surface motif — an acidic loop followed by a long β-hairpin — that is recognized by TLR11 to trigger IL-12 secretion in macrophages; deleting both elements abolishes IL-12 induction, and grafting them onto yeast profilin is sufficient to confer TLR11-dependent signaling, establishing this motif as the key molecular pattern (PMID:20851125). Biochemically, TgPRF diverges from vertebrate profilins: it binds monomeric actin weakly and slows rather than accelerates nucleotide exchange, with a putative actin-binding surface that includes the β-hairpin (PMID:20851125). Beyond TLR11, TgPRF activates the NLRP3 inflammasome in macrophages, driving processing and secretion of IL-1β in a manner blocked by the NLRP3 inhibitor MCC950, and it upregulates TLR5 expression (PMID:37080500).

1. 2010 High

   Defining the structural basis by which the host innate immune system detects a parasite profilin, this work identified an acidic loop/β-hairpin motif as the molecular pattern recognized by TLR11 to drive IL-12 secretion.

   Evidence X-ray crystallography with structure-based mutagenesis, IL-12 secretion assays in peritoneal macrophages, and gain-of-function chimeric insertion into yeast profilin

   PMID:20851125

   Open questions at the time

   • Does not resolve the TLR11 binding interface at atomic resolution
   • IL-12 shown in mouse peritoneal macrophages; relevance to human TLR biology not addressed here
2. 2010 Medium

   To test whether TgPRF retains canonical profilin actin-regulatory activity, biochemical assays showed it behaves unlike vertebrate profilins — binding actin weakly and slowing nucleotide exchange — indicating a divergent actin-binding surface.

   Evidence In vitro nucleotide exchange and actin-binding assays with monomeric rabbit actin plus structural comparison

   PMID:20851125

   Open questions at the time

   • Actin-binding surface not confirmed by mutagenesis
   • Assays used rabbit actin, not parasite actin; physiological relevance to gliding motility not directly tested
3. 2023 Medium

   Extending TgPRF's immunostimulatory role beyond TLR11/IL-12, this work showed it also engages the NLRP3 inflammasome to drive IL-1β production, revealing a second innate immune axis.

   Evidence NLRP3 inhibitor (MCC950) treatment with western blot, ELISA, and qRT-PCR in THP-1 macrophages

   PMID:37080500

   Open questions at the time

   • Mechanism linking TgPRF to NLRP3 activation not defined
   • TLR5 upregulation observed but its causal role in inflammasome activation not established
   • Single cell-line study without genetic knockout confirmation

• How TgPRF's actin-regulatory activity mechanistically contributes to parasite gliding motility, and how its immune-detection functions integrate in vivo, remain unresolved.
• No direct in vivo link between TgPRF actin binding and gliding motility in the corpus
• Physical TgPRF–TLR11 and TgPRF–NLRP3 interactions not structurally defined