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Role of the Plasmodium export element in trafficking parasite proteins to the infected erythrocyte.

Boddey JA, Moritz RL, Simpson RJ, Cowman AF - Traffic (2008)

Bottom Line: The PEXEL constitutes a bifunctional export motif comprising a protease recognition sequence that is cleaved, in the endoplasmic reticulum, from proteins destined for export, in a PEXEL arginine- and leucine-dependent manner.Following processing, the remaining conserved PEXEL residue is required to direct the mature protein to the host cell.Furthermore, we demonstrate that N acetylation of proteins following N-terminal processing is a PEXEL-independent process that is insufficient for correct export to the host cell.

View Article: PubMed Central - PubMed

Affiliation: The Walter and Eliza Hall Institute of Medical Research, Parkville 3050, Melbourne, Australia.

ABSTRACT
The intracellular survival of Plasmodium falciparum within human erythrocytes is dependent on export of parasite proteins that remodel the host cell. Most exported proteins require a conserved motif (RxLxE/Q/D), termed the Plasmodium export element (PEXEL) or vacuolar targeting sequence (VTS), for targeting beyond the parasitophorous vacuole membrane and into the host cell; however, the precise role of this motif in export is poorly defined. We used transgenic P. falciparum expressing chimeric proteins to investigate the function of the PEXEL motif for export. The PEXEL constitutes a bifunctional export motif comprising a protease recognition sequence that is cleaved, in the endoplasmic reticulum, from proteins destined for export, in a PEXEL arginine- and leucine-dependent manner. Following processing, the remaining conserved PEXEL residue is required to direct the mature protein to the host cell. Furthermore, we demonstrate that N acetylation of proteins following N-terminal processing is a PEXEL-independent process that is insufficient for correct export to the host cell. This work defines the role of each residue in the PEXEL for export into the P. falciparum-infected erythrocyte.

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Related in: MedlinePlus

Role of the PEXEL in export of Plasmodium falciparum proteins to the infected erythrocyteTwo proposedmodels of PEXEL-mediated export are shown (A, 1 and 2). A1) After cotranslational insertion through Sec61 at the rough endoplasmic reticulum (rER) using the signal sequence, proteins to be exported are either processed by signal peptidase (red pac-man) or sequestered and/or processed by the PEXEL protease (yellow pac-man) and sorted at the transitional endoplasmic reticulum (tER) for transport through the Golgi to the parasitophorous vacuole by the default secretory pathway. There, proteins to be exported (xE/Q/D after PEXEL processing; green protein), are recognised and trafficked across the parasitophorous vacuole membrane by a translocon. Secreted or mutated PEXEL proteins are depicted as red proteins. A2) After entry at the rER and sequestration and/or processing by either signal peptidase (red pac-man) or the PEXEL protease (yellow pac-man), proteins to be exported are differentially sorted either at the tER or at the Golgi into vesicles. This may occur through a specific transmembrane PEXEL cargo receptor that enriches functionally distinct vesicles for exported proteins (green proteins), which are targeted to subcompartments (the ‘necklace of beads’; depicted as white compartments in the parasitophorous vacuole) of the parasitophorous vacuole that houses the translocon. Exported transmembrane proteins then presumably diffuse laterally from the translocon and traffic with forming Maurer's clefts (white structures in the erythrocyte). Secreted proteins (red) traffic through the default secretory pathway to alternative compartments of the parasitophorous vacuole that do not contain the translocon. The default pathway may involve bulk flow, depicted as free red proteins in the ER that ‘sample’ the budding membrane. Uncharacterised vesicles are depicted by ‘?’. Close up (box) of the possible sorting mechanism is shown in (B) and (C). B) After cotranslational insertion into the rER PEXEL proteins are sequestered/processed by the PEXEL protease and sorted into vesicles through a transmembrane cargo receptor that interacts with the COPII machinery. C) Secreted proteins are not recognised by the PEXEL receptor but bind either a different receptor or traffic through bulk flow. The role of the Golgi is unclear but similar sorting may occur there.
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fig09: Role of the PEXEL in export of Plasmodium falciparum proteins to the infected erythrocyteTwo proposedmodels of PEXEL-mediated export are shown (A, 1 and 2). A1) After cotranslational insertion through Sec61 at the rough endoplasmic reticulum (rER) using the signal sequence, proteins to be exported are either processed by signal peptidase (red pac-man) or sequestered and/or processed by the PEXEL protease (yellow pac-man) and sorted at the transitional endoplasmic reticulum (tER) for transport through the Golgi to the parasitophorous vacuole by the default secretory pathway. There, proteins to be exported (xE/Q/D after PEXEL processing; green protein), are recognised and trafficked across the parasitophorous vacuole membrane by a translocon. Secreted or mutated PEXEL proteins are depicted as red proteins. A2) After entry at the rER and sequestration and/or processing by either signal peptidase (red pac-man) or the PEXEL protease (yellow pac-man), proteins to be exported are differentially sorted either at the tER or at the Golgi into vesicles. This may occur through a specific transmembrane PEXEL cargo receptor that enriches functionally distinct vesicles for exported proteins (green proteins), which are targeted to subcompartments (the ‘necklace of beads’; depicted as white compartments in the parasitophorous vacuole) of the parasitophorous vacuole that houses the translocon. Exported transmembrane proteins then presumably diffuse laterally from the translocon and traffic with forming Maurer's clefts (white structures in the erythrocyte). Secreted proteins (red) traffic through the default secretory pathway to alternative compartments of the parasitophorous vacuole that do not contain the translocon. The default pathway may involve bulk flow, depicted as free red proteins in the ER that ‘sample’ the budding membrane. Uncharacterised vesicles are depicted by ‘?’. Close up (box) of the possible sorting mechanism is shown in (B) and (C). B) After cotranslational insertion into the rER PEXEL proteins are sequestered/processed by the PEXEL protease and sorted into vesicles through a transmembrane cargo receptor that interacts with the COPII machinery. C) Secreted proteins are not recognised by the PEXEL receptor but bind either a different receptor or traffic through bulk flow. The role of the Golgi is unclear but similar sorting may occur there.

Mentions: While cleavage of the PEXEL motif at leucine, revealing xE/Q/D at the resulting N-terminus, is a requirement for export to the erythrocyte, the route and machinery involved are unknown. Endomembrane protein transport involves trafficking through protein-coated vesicles to target membranes. Polymerisation of COPII proteins at the transitional ER generates vesicles for transport from the ER and the heptameric COPI complex, along with ARF1, generates vesicles for reterograde transport (reviewed in 34). To achieve sorting, transmembrane cargo can interact directly with coat proteins; however, soluble cargo requires transmembrane receptors (reviewed in 35). Alternatively, proteins may traffic indirectly by bulk flow (36). Plasmodium falciparum contains all the COPII subunits (37–41) and at least four COPI subunits (alpha, beta, delta and epsilon) (42,43); however, the exact nature of vesicles that fuse with the parasite membrane is unknown. There are two likely pathways for export of proteins from the ER to the parasite-infected erythrocyte (Figure 9). In the first model, proteins to be exported are cotranslationally inserted into the ER membrane using the signal sequence, allowing cleavage by signal peptidase and/or recognition of the PEXEL by a specific protease that cleaves this motif. This reveals xE/Q/D at the N-terminus of the protein, which is trafficked to the Golgi and parasitophorous vacuole through the general secretory pathway using the COPII machinery. Once released into the parasitophorous vacuole a translocon, that has been hypothesised previously (2,19,31,44), would recognise the cleaved motif for export to the erythrocyte. While this model is possible, it would seem less likely as some proteins that are not exported, but may be secreted into the parasitophorous vacuole, would have xE/Q/D revealed after cleavage of the signal sequence (Boddey and Cowman, unpublished), and these mature proteins would be indistinguishable from those destined for export.


Role of the Plasmodium export element in trafficking parasite proteins to the infected erythrocyte.

Boddey JA, Moritz RL, Simpson RJ, Cowman AF - Traffic (2008)

Role of the PEXEL in export of Plasmodium falciparum proteins to the infected erythrocyteTwo proposedmodels of PEXEL-mediated export are shown (A, 1 and 2). A1) After cotranslational insertion through Sec61 at the rough endoplasmic reticulum (rER) using the signal sequence, proteins to be exported are either processed by signal peptidase (red pac-man) or sequestered and/or processed by the PEXEL protease (yellow pac-man) and sorted at the transitional endoplasmic reticulum (tER) for transport through the Golgi to the parasitophorous vacuole by the default secretory pathway. There, proteins to be exported (xE/Q/D after PEXEL processing; green protein), are recognised and trafficked across the parasitophorous vacuole membrane by a translocon. Secreted or mutated PEXEL proteins are depicted as red proteins. A2) After entry at the rER and sequestration and/or processing by either signal peptidase (red pac-man) or the PEXEL protease (yellow pac-man), proteins to be exported are differentially sorted either at the tER or at the Golgi into vesicles. This may occur through a specific transmembrane PEXEL cargo receptor that enriches functionally distinct vesicles for exported proteins (green proteins), which are targeted to subcompartments (the ‘necklace of beads’; depicted as white compartments in the parasitophorous vacuole) of the parasitophorous vacuole that houses the translocon. Exported transmembrane proteins then presumably diffuse laterally from the translocon and traffic with forming Maurer's clefts (white structures in the erythrocyte). Secreted proteins (red) traffic through the default secretory pathway to alternative compartments of the parasitophorous vacuole that do not contain the translocon. The default pathway may involve bulk flow, depicted as free red proteins in the ER that ‘sample’ the budding membrane. Uncharacterised vesicles are depicted by ‘?’. Close up (box) of the possible sorting mechanism is shown in (B) and (C). B) After cotranslational insertion into the rER PEXEL proteins are sequestered/processed by the PEXEL protease and sorted into vesicles through a transmembrane cargo receptor that interacts with the COPII machinery. C) Secreted proteins are not recognised by the PEXEL receptor but bind either a different receptor or traffic through bulk flow. The role of the Golgi is unclear but similar sorting may occur there.
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Related In: Results  -  Collection

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fig09: Role of the PEXEL in export of Plasmodium falciparum proteins to the infected erythrocyteTwo proposedmodels of PEXEL-mediated export are shown (A, 1 and 2). A1) After cotranslational insertion through Sec61 at the rough endoplasmic reticulum (rER) using the signal sequence, proteins to be exported are either processed by signal peptidase (red pac-man) or sequestered and/or processed by the PEXEL protease (yellow pac-man) and sorted at the transitional endoplasmic reticulum (tER) for transport through the Golgi to the parasitophorous vacuole by the default secretory pathway. There, proteins to be exported (xE/Q/D after PEXEL processing; green protein), are recognised and trafficked across the parasitophorous vacuole membrane by a translocon. Secreted or mutated PEXEL proteins are depicted as red proteins. A2) After entry at the rER and sequestration and/or processing by either signal peptidase (red pac-man) or the PEXEL protease (yellow pac-man), proteins to be exported are differentially sorted either at the tER or at the Golgi into vesicles. This may occur through a specific transmembrane PEXEL cargo receptor that enriches functionally distinct vesicles for exported proteins (green proteins), which are targeted to subcompartments (the ‘necklace of beads’; depicted as white compartments in the parasitophorous vacuole) of the parasitophorous vacuole that houses the translocon. Exported transmembrane proteins then presumably diffuse laterally from the translocon and traffic with forming Maurer's clefts (white structures in the erythrocyte). Secreted proteins (red) traffic through the default secretory pathway to alternative compartments of the parasitophorous vacuole that do not contain the translocon. The default pathway may involve bulk flow, depicted as free red proteins in the ER that ‘sample’ the budding membrane. Uncharacterised vesicles are depicted by ‘?’. Close up (box) of the possible sorting mechanism is shown in (B) and (C). B) After cotranslational insertion into the rER PEXEL proteins are sequestered/processed by the PEXEL protease and sorted into vesicles through a transmembrane cargo receptor that interacts with the COPII machinery. C) Secreted proteins are not recognised by the PEXEL receptor but bind either a different receptor or traffic through bulk flow. The role of the Golgi is unclear but similar sorting may occur there.
Mentions: While cleavage of the PEXEL motif at leucine, revealing xE/Q/D at the resulting N-terminus, is a requirement for export to the erythrocyte, the route and machinery involved are unknown. Endomembrane protein transport involves trafficking through protein-coated vesicles to target membranes. Polymerisation of COPII proteins at the transitional ER generates vesicles for transport from the ER and the heptameric COPI complex, along with ARF1, generates vesicles for reterograde transport (reviewed in 34). To achieve sorting, transmembrane cargo can interact directly with coat proteins; however, soluble cargo requires transmembrane receptors (reviewed in 35). Alternatively, proteins may traffic indirectly by bulk flow (36). Plasmodium falciparum contains all the COPII subunits (37–41) and at least four COPI subunits (alpha, beta, delta and epsilon) (42,43); however, the exact nature of vesicles that fuse with the parasite membrane is unknown. There are two likely pathways for export of proteins from the ER to the parasite-infected erythrocyte (Figure 9). In the first model, proteins to be exported are cotranslationally inserted into the ER membrane using the signal sequence, allowing cleavage by signal peptidase and/or recognition of the PEXEL by a specific protease that cleaves this motif. This reveals xE/Q/D at the N-terminus of the protein, which is trafficked to the Golgi and parasitophorous vacuole through the general secretory pathway using the COPII machinery. Once released into the parasitophorous vacuole a translocon, that has been hypothesised previously (2,19,31,44), would recognise the cleaved motif for export to the erythrocyte. While this model is possible, it would seem less likely as some proteins that are not exported, but may be secreted into the parasitophorous vacuole, would have xE/Q/D revealed after cleavage of the signal sequence (Boddey and Cowman, unpublished), and these mature proteins would be indistinguishable from those destined for export.

Bottom Line: The PEXEL constitutes a bifunctional export motif comprising a protease recognition sequence that is cleaved, in the endoplasmic reticulum, from proteins destined for export, in a PEXEL arginine- and leucine-dependent manner.Following processing, the remaining conserved PEXEL residue is required to direct the mature protein to the host cell.Furthermore, we demonstrate that N acetylation of proteins following N-terminal processing is a PEXEL-independent process that is insufficient for correct export to the host cell.

View Article: PubMed Central - PubMed

Affiliation: The Walter and Eliza Hall Institute of Medical Research, Parkville 3050, Melbourne, Australia.

ABSTRACT
The intracellular survival of Plasmodium falciparum within human erythrocytes is dependent on export of parasite proteins that remodel the host cell. Most exported proteins require a conserved motif (RxLxE/Q/D), termed the Plasmodium export element (PEXEL) or vacuolar targeting sequence (VTS), for targeting beyond the parasitophorous vacuole membrane and into the host cell; however, the precise role of this motif in export is poorly defined. We used transgenic P. falciparum expressing chimeric proteins to investigate the function of the PEXEL motif for export. The PEXEL constitutes a bifunctional export motif comprising a protease recognition sequence that is cleaved, in the endoplasmic reticulum, from proteins destined for export, in a PEXEL arginine- and leucine-dependent manner. Following processing, the remaining conserved PEXEL residue is required to direct the mature protein to the host cell. Furthermore, we demonstrate that N acetylation of proteins following N-terminal processing is a PEXEL-independent process that is insufficient for correct export to the host cell. This work defines the role of each residue in the PEXEL for export into the P. falciparum-infected erythrocyte.

Show MeSH
Related in: MedlinePlus