BIOSYNTHESIS, EXPORT AND PROCESSING OF A 45 KDA PROTEIN DETECTED IN MEMBRANE CLEFTS OF ERYTHROCYTES INFECTED WITH PLASMODIUM-FALCIPARUM

During its asexual life cycle, the human malaria parasite Plasmodium f alciparum exports numerous proteins beyond its surface to its host ery throcyte. We have studied the biosynthesis, processing and export of a 45 kDa parasite protein resident in membrane clefts in the erythrocyt e cytoplasm. Our results indicate that this cleft protein is made as a single tightly membrane-bound 45 kDa polypeptide in ring- and trophoz oite-infected erythrocytes (0-36 h in the life cycle). Using ring/trop hozoite parasites released from erythrocytes, the 45 kDa protein is sh own to be efficiently transported to the cell surface. This export is specifically blocked by the drug brefeldin A, and at 15 and 20 degrees C. These results indicate that transport blocks seen in the Golgi of mammalian cells are conserved in P. falciparum. Further, the newly syn thesized 45 kDa protein passes through parasite Golgi compartments bef ore its export to clefts in the erythrocyte. In mid-to-late-ring-infec ted erythrocytes, a fraction of the newly synthesized 45 kDa protein i s processed to a second membrane-bound phosphorylated 47 kDa protein. The t1/2 of this processing step is about 4 h, suggesting that it occu rs subsequent to protein export from the parasite. Evidence is present ed that, in later trophozoite stages (26-36 h), the exported 45 and 47 kDa proteins are partially converted into soluble molecules in the in traerythrocytic space. Taken together, the results indicate that the l ower eukaryote P. falciparum modulates a classical secretory pathway t o support membrane export beyond its plasma membrane to clefts in the erythrocyte. Subsequent to export, phosphorylation and/or conversion i nto a soluble form may regulate the interactions of the 45 kDa protein with the clefts during parasite development.