Die biochemische Analyse des Plasmodium falciparum Zytoadhärenz Moleküls PfEmp1 zeigt einen potentiell neuen Mechanismus für die Insertion von Oberflächenproteinen in Membranen

The Plasmodium falciparum cytoadherence factor PfEmp1 is expressed on the surface of red blood cells where it interacts with various endothelial receptors leading to the sequestration of infected red blood cells in blood capillaries. In addition, PfEmp1 undergoes antigenic variation giving rise to the generation of irrelevant antibodies in the immune response. Thus PfEmp1 is a major factor which contributes to the severe pathology of malaria. PfEmp1 possesses a hydrophobic segment in the vicinity of the carboxy end which according to a commonly accepted view constitutes a transmembrane helix anchoring the protein into the erythrocyte membrane. This model provides a protein secretion machinery in red blood cells, which by themselves are incapable of the synthesis and trafficking of proteins. In this thesis the transport model on PfEmp1 was issued by a precise biochemical examination resulting in to the general view on PfEmp1 trafficking conflicting data that suggest a novel model on synthesis and transport of the protein. After treatment of infected cells with the protein secretion blocking agent brefeldin A PfEmp1 was retained in the parasite. In contrast to integral membrane proteins the protein could be extracted with alkaline carbonate buffer, typical of peripheral membrane proteins. Another goal of this thesis was the development of an assay that allows to investigate transport vesicles such as suggested to be involved in the secretion of PfEmp1. No association of the protein with vesicles was found. Once the protein had been secreted into the host cell it could be extracted under conditions that left integral membrane proteins insoluble. PfEmp1 of the red cell membrane could be extracted with urea which integral membrane proteins of the red cell membrane resisted. Further, PfEmp1 could be separated from these marker membrane proteins by density gradient centrifugation after urea treatment. The results of this thesis imply a novel picture of the membrane association of PfEmp1. The protein presumably becomes secreted as a peripheral membrane protein or as part of a protein complex. By an unknown mechanism on the basis of protein interaction the protein is targeted to the red cell surface where it is inserted into the membrane keeping the hydrophobic segment of the protein bound to proteins.