The aspartyl protease TgASP5 mediates the export of theToxoplasmaGRA16 and GRA24 effectors into host cells

Abstract

Toxoplasma gondii and Plasmodium species are obligatory intracellular parasites that export proteins into the infected cells in order to interfere with host-signalling pathways, acquire nutrients or evade host defense mechanisms. With regard to export mechanism, a wealth of information in Plasmodium spp. is available, while the mechanisms operating in T. gondii remain uncertain. The recent discovery of exported proteins in T. gondii, mainly represented by dense granule resident proteins, might explain this discrepancy and offers a unique opportunity to study the export mechanism in T. gondii. Here, we report that GRA16 export is mediated by two protein elements present in its N-terminal region. Because the first element contains a putative Plasmodium export element linear motif (RRLAE), we hypothesized that GRA16 export depended on a maturation process involving protein cleavage. Using both N- and C-terminal epitope tags, we provide evidence for protein proteolysis occurring in the N-terminus of GRA16. We show that TgASP5, the T. gondii homolog of Plasmodium plasmepsin V, is essential for GRA16 export and is directly responsible for its maturation in a Plasmodium export element-dependent manner. Interestingly, TgASP5 is also involved in GRA24 export, although the GRA24 maturation mechanism is TgASP5-independent. Our data reveal different modus operandi for protein export, in which TgASP5 should play multiple functions.

The RxLR Motif of the Host Targeting Effector AVR3a of Phytophthora infestans Is Cleaved before Secretion

“…The processing of native AVR3a secreted from P. infestans is indeed remarkably similar to the cleavage observed for the PEXEL motif of effectors from P. falciparum (Chang et al, 2008;Boddey et al, 2009;Russo et al, 2010) (Figure 4) and also for the TEXEL motif of effectors from T. gondii (Coffey et al, 2015;Curt-Varesano et al, 2016). However, the RxLR motif is not recognized and cleaved by the ER-located plasmepsin V protease of P. falciparum (Boddey et al, 2016).…”

“…The conserved spatial position of the PEXEL motif and its very rapid proteolytic modification are necessary for the export of effector proteins and seem to work as an internal signal for a specialized effector export pathway (Marti and Spielmann, 2013;Boddey et al, 2016). Recently, a similar motif (TEXEL) was found in effector proteins of Toxoplasma gondii and reported to play a crucial role in effector maturation and sorting during the infection process (Hammoudi et al, 2015;Coffey et al, 2015;Curt-Varesano et al, 2016).…”

Secreted protein kinases regulate cyst burden during chronic toxoplasmosis

“…Although the PEXEL motif was initially defined in Plasmodium falciparum (Hiller et al, ; Marti, Good, Rug, Knuepfer, & Cowman, ), T. gondii utilizes similar motifs in many of the secreted dense granule (GRA) proteins. A similar TEXEL motif is processed by the Golgi‐localized ASP5 protease (Coffey et al, ; Curt‐Varesano, Braun, Ranquet, Hakimi, & Bougdour, ; Hammoudi et al, ). By motif searching in CLC Genomics software, the following potential HT/PEXEL and TEXEL sites were identified in ROP21 : RQLLE 111–115 and RRLAG 250–254 , ROP27: RRLKT 443–447 , and ROP28: RRLLR 55–59 .…”

Toxoplasma Effector Recruits the Mi-2/NuRD Complex to Repress STAT1 Transcription and Block IFN-γ-Dependent Gene Expression

“…This cellular distribution of TgIST is similar to other recently described T. gondii effectors such as GRA16 and GRA24 that affect p38 MAPK signaling (Braun et al, 2013) and p53 (Bougdour et al, 2013), respectively. TgIST contains features that suggest it might be processed by the aspartic protease TgASP5, which has recently been shown to be important for export of a number of exported dense granule effectors (Curt-Varesano et al, 2015; Hammoudi et al, 2015). Moreover, the absence of the parasite effector MYR1, a distinct protein involved in export of dense granule effectors (Franco et al, 2016), phenocopies the defect of TgIST in blocking IRF1 induction by IFN-γ, suggesting it may affect export of TgIST.…”