Plasmodium falciparum: glycosylation status of Plasmodium falciparum circumsporozoite protein expressed in the baculovirus system

Abstract

We expressed the main surface antigen of Plasmodium falciparum sporozoites, the circumsporozoite protein (CSP), in High Five ( Trichoplusia ni) insect cells using the baculovirus system. Significant amounts of the recombinant protein could be obtained, as judged by SDS–PAGE, Western blot, and immunofluorescence analysis. The cellular localization for recombinant CSP was determined by immunofluorescence. The high fluorescence signal of the permeabilized cells, relative to that of fixed nonpermeabilized cells, revealed a clear intracellular localization of this surface antigen. Analysis of possible posttranslational modifications of CSP showed that this recombinant protein is only N-glycosylated in the baculovirus system. Although DNA-sequence analysis revealed a GPI-cleavage/attachment site, no GPI anchor could be demonstrated. These analyses show that the glycosylation status of this recombinant protein may not reflect its native form in P. falciparum. The impact of these findings on vaccine development will be discussed. Index descriptors and abbreviations: Glycosylphosphatidylinositol; Circumsporozoite; Insect cells; Baculovirus; ER, endoplasmic reticulum; ETL, early-to-late; GPI, glycosylphosphatidylinositol; mAb, monoclonal antibody; CSP, circumsporozoite protein; IFA, indirect immunofluorescence assay; m.o.i., multiplicity of infection; PBS, phosphate-buffered saline; p.i., postinfection; PI-PLC, phosphatidylinositol-specific phospholipase C; MSP-1, merozoite surface protein 1; SDS–PAGE, sodium dodecyl sulfate–polyacrylamide gel electrophoresis.