Structure-Based Analysis of Toxoplasma gondii Profilin: A Parasite-Specific Motif Is Required for Recognition by Toll-Like Receptor 11

Abstract

Profilins promote actin polymerization by exchanging ADP for ATP on monomeric actin, and delivering ATP-actin to growing filament barbed ends. Apicomplexan protozoa like Toxoplasma gondii invade host cells using an actin-dependent gliding motility. Toll-like receptor 11 (TLR11) generates an innate immune response upon sensing T. gondii profilin (TgPRF). The crystal structure of TgPRF reveals a parasite-specific surface motif consisting of an acidic loop, followed by a long β-hairpin. A series of structure-based profilin mutants show that TLR11 recognition of the acidic loop is responsible for most of the IL-12 secretion response to TgPRF in peritoneal macrophages. Deletion of both the acidic loop and the β-hairpin completely abrogates IL-12 secretion. Insertion of the T. gondii acidic loop and β-hairpin into yeast profilin is sufficient to generate TLR11-dependent signaling. Substitution of the acidic loop in TgPRF with the homologous loop from the apicomplexan parasite C. parvum does not affect TLR11-dependent IL-12 secretion, while substitution with the acidic loop from P. falciparum results in reduced but significant IL-12 secretion. We conclude that the parasite-specific motif in TgPRF is the key molecular pattern recognized by TLR11. Unlike other profilins, TgPRF slows nucleotide exchange on monomeric rabbit actin, and binds rabbit actin weakly. The putative TgPRF actin-binding surface includes the β-hairpin, and diverges widely from the actin-binding surfaces of vertebrate profilins.

The Lasso Segment Is Required for Functional Dimerization of the Plasmodium Formin 1 FH2 Domain

“…In Pf -Pfn, the aromatic nature of the proline-rich-peptide binding site is conserved but many of the exact amino acid positions are not [7]. The same is true for T. gondii profilin [47], and furthermore, there are differences in the aromatic residues of the peptide-binding surface of these two apicomplexan profilins, as well. Therefore, it is possible that they have somewhat divergent functions and mechanisms.…”

Toxoplasma gondii Profilin Acts Primarily To Sequester G-Actin While Formins Efficiently Nucleate Actin Filament Formation in Vitro

“…Despite interacting with heterologous actin in vitro , TgPRF is unable to complement depletion of profilin in yeast (25). Additionally, opposite of conventional profilins, TgPRF was shown to inhibit nucleotide exchange by rabbit actin (26). Finally, profilin in the apicomplexan parasite P. falciparum is essential in the blood-stage of the parasite life cycle (27) and functionally complements the depletion of PRF in T. gondii (25).…”

Recombinant production, crystallization and preliminary structural characterization ofSchistosoma japonicumprofilin

“…Other parasites, such as the Apicomplexa, also possess highly divergent profilins which still bind to actin, proline-rich sequence motifs and phosphoinositides (Kursula et al, 2008;Kucera et al, 2010). However, certain other profilins, such as mammalian profilin 4, do not bind to actin or proline-rich sequences despite their overall conserved fold (Behnen et al, 2009).…”