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An alternative secretory pathway in the malaria parasite Plasmodium falciparum
The malaria parasite P. falciparum invades human red blood cells (RBCs). During invasion a compartment surrounding the parasite, the so-called parasitophorous vacuole (PV), is formed. The parasite resides and develops within the PV, which protects the parasite from the host cell cytosol. During i...
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| Format: | Doctoral Thesis |
| Language: | English |
| Published: |
Philipps-Universität Marburg
2014
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| Online Access: | PDF Full Text |
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| Summary: | The malaria parasite P. falciparum invades human red blood cells (RBCs). During
invasion a compartment surrounding the parasite, the so-called parasitophorous vacuole
(PV), is formed. The parasite resides and develops within the PV, which protects the
parasite from the host cell cytosol. During its intraerythrocytic growth the parasite
exports a vast number of proteins to the host cell in order to maintain its survival within
the RBC. Proteins, which are directed to the host cell cytosol and host cell membrane,
respectively, therefore are challenged to cross the parasite plasma membrane, the PV
and the parasitophorous vacuolar membrane (PVM). However, the secretion and export
mechanisms of many parasite proteins are still not understood.
The current study focuses on the discovery of an alternative secretory pathway to the
ER/Golgi route in the malaria parasite P. falciparum in infected RBCs. Two proteins
appeared to be promising candidates of an alternative secretory pathway: the PfADPribosylation
factor 1 (ARF1) and the Pfadenylate kinase 2 (AK2). Both proteins
contained a N-myristoylation site at their N-terminus, which is indicative for Nmyristoylation.
N-myristoylation is a co-translational modification of a protein, whereby
a fatty acid (myristate) is irreversibly attached to the glycine residue at the N-terminus
of a protein via the PfN-myristoyltransferase (NMT). A preceding proteomic analysis of
the parasitophorous vacuole and a reporter construct study proposed for both PfARF1
(determined by a proteomic study) and PfAK2 (determined by a reporter construct
study) PV localization although both proteins lacked a signal peptide. That’s why it was
hypothesized whether or not N-myristoylation would drive protein secretion across the
parasite plasma membrane (PPM). The subcellular localization of the PfARF1/GFP
parasites and the PfAK2/GFP parasites, respectively, were analyzed via epifluorescence
microscopy and biochemical methods. In parallel, another batch of reporter constructs
were generated and analyzed, where the N-myristoylation site of PfARF1 (this study)
and PfAK2 (Ma et al., 2012), respectively, was removed (PfARF1G2A/GFP and
PfAK2G2A/GFP). Live cell imaging showed that the fusion protein ARF1/GFP was
localized as dot-like structures in the parasite. In contrast, the phenotype of the fusion protein of the PfARF1G2A/GFP parasites showed an evenly distributed signal in the
parasite cytosol. Further analysis of the subcellular localization of the PfARF1 strongly
supports its localization to compartments of the early secretory pathway of the parasite,
but no localization in the PV. In contrast, the fusion protein PfAK2/GFP localized to a
ring-like structure around the parasite indicating PV localization. The PfAK2G2A/GFP
parasites showed a cytosolic localization of the fusion protein (Ma et al., 2012).
Biochemical analyses revaled that the fusion protein PfAK2/GFP was secreted into the
PV when the N-myristoylation site was present. Furthermore, it could be shown that the
N-terminus of the PfAK2 protein is sufficient for parasite plasma membrane targeting,
stable membrane anchoring and subsequent protein translocation across the PPM. A
possible role of the early secretory pathway in PfAK2 trafficking and the folding state
of PfAK2 prior to translocation across the PPM was also examined. However, the exact
mechanism how PfAK2 is translocated across the PPM still remains elusive. |
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| DOI: | 10.17192/z2014.0238 |