An erythrocyte cytoskeleton-binding motif in exported Plasmodium falciparum proteins.

Binding of exported malaria parasite proteins to the host cell membrane and cytoskeleton contributes to the morphological, functional, and antigenic changes seen in Plasmodium falciparum-infected erythrocytes. One such exported protein that targets the erythrocyte cytoskeleton is the mature parasite-infected erythrocyte surface antigen (MESA), which interacts with the N-terminal 30-kDa domain of protein 4.1R via a 19-residue sequence. We report here that the MESA erythrocyte cytoskeleton-binding (MEC) domain is present in at least 13 other P. falciparum proteins predicted to be exported to the host cell. An alignment of the putative cytoskeleton-binding sequences revealed a conserved aspartic acid at the C terminus that was omitted from the originally reported binding domain. Mutagenesis experiments demonstrated that this aspartic acid was required for the optimal binding of MESA to inside-out vesicles (IOVs) prepared from erythrocytes. Using pulldown assays, we characterized the binding of fragments encoding the MEC domains from PFE0040c/MESA and six other proteins (PF10_0378, PFA0675w, PFB0925w, PFD0095c, PFF1510w, and PFI1790w) to IOVs. All seven proteins bound to IOVs, with MESA showing the strongest affinity in saturation binding experiments. We further examined the interaction of the MEC domain proteins with components of the erythrocyte cytoskeleton and showed that MESA, PF10_0378, and PFA0675w coprecipitated full-length 4.1R from lysates prepared from IOVs. These data demonstrated that the MEC motif is present and functional in at least six other P. falciparum proteins that are exported to the host cell cytoplasm. The pathology of the malaria parasite Plasmodium falciparum is associated with its ability to remodel the red blood cells (RBCs) it infects. Among the most dramatic changes induced by P. falciparum is the formation of thousands of protrusions (knobs) on the RBC surface, into which the P. falciparum erythrocyte membrane protein 1 (PfEMP1) is inserted (11, 12, 17). PfEMP1 is an antigenically variant protein that acts as a receptor for host ligands located on the endothelial lining of the vasculature or on other RBCs. Binding to endothelial cells sequesters infected RBCs in the microvasculature, which prevents the clearance of the infected RBC by the spleen and contributes to the pathogenesis of severe malaria (54). Other changes to the infected RBC include the elaboration of Golgi membrane-like vesicle stacks called Maurer’s clefts and a membrane-bound tubovesicular network that together promote trafficking of molecules to and from the RBC surface (27, 29). In addition to the ultrastructural changes, the functional properties of the infected RBC membrane also are affected. A new permeability pathway, which enables the transport of a range of nutrients and small molecules, is established through the insertion of parasite proteins into the RBC membrane (39, 50). The mechanical properties of the RBC membrane are altered, leading to decreased deformability and increased rigidity (37). Even the geometry of the RBC is affected, as indicated by an increase in the minimal cylindrical diameter,