A homologue of Sar1p localises to a novel trafficking pathway in malaria-infected erythrocytes.

We have identified a homologue of the GTP-binding protein, Sar1p, in Plasmodium falciparum. Sar1p is a small GTPase that is thought to play a crucial role in trafficking of proteins between the endoplasmic reticulum and the Golgi. The P.falciparum SAR1 gene is located on chromosome 4 and comprises two exons separated by a 508 bp intron. The deduced amino acid sequence of PfSar1p (GenBank accession number AF104306) shows 71% similarity (58% identity) to Sar1p from Saccharomyces cerevisiae. Expression of PfSar1p in erythrocytic stages of P. falciparum was confirmed by sequencing of a tryptic peptide derived from a polypeptide excised from an SDS-polyacrylamide gel. A recombinant protein corresponding to approximately 70% of the PfSar1p sequence was used to raise antibodies. The affinity-purified antiserum recognised a protein with an apparent molecular weight of 23 K in Western blots of malaria-infected erythrocytes but not in uninfected erythrocytes. PfSar1p was shown to be largely insoluble in non-ionic detergent and a low ionic strength buffer. Confocal immunofluorescence microscopy of malaria-infected erythrocytes was used to show that PfSar1p is located near the periphery of the parasite in discrete compartments, which appear to be distinct from the parasite endoplasmic reticulum. In addition, PfSar1p appears to be exported to structures outside the parasite in the erythrocyte cytoplasm. The export of PfSar1p to the erythrocyte cytosol is inhibited by treatment with brefeldin A. This provides the first evidence that the malaria parasite is capable of elaborating components of the classical vesicle-mediated trafficking machinery outside the boundaries of its own plasma membrane.