Plasmodium falciparum malaria: association of knobs on the surface of infected erythrocytes with a histidine-rich protein and the erythrocyte skeleton.

Abstract

P. falciparum-infected erythrocytes (RBC) develop surface protrusions (knobs) which consist of electron-dense submembrane cups and the overlying RBC plasma membrane. Knobs mediate cytoadherence to endothelial cells. Falciparum variants exist that lack knobs. Using knobby (K+) and knobless (K-) variants of 2 strains of P. falciparum, Kilejian''s original observation that a histidine-rich protein occurred in K+ parasites but not K- variants (Kilejian 1979 and Kilejian, 1980) was confirmed. Two additional histidine-rich proteins of lower MW were synthesized by K+ and K- variants of both strains. Differential detergent extraction and thin-section EM were used to investigate the subcellular location of the histidine-rich protein unique to K+ parasites. Triton X-100, Zwittergent 314, cholic acid, CHAPS [3-[(3-cholamidopropyl)dimethylammonio]-1-propane-sulfonate] and Triton X-100/0.6 M KCl failed to extract the unique histidine-rich protein. The residues insoluble in these detergents contained the unique histidine-rich protein and electron-dense cups. The protein was extracted by 1% SDS [sodium dodecyl sulfate] and by 1% Triton X-100/9 M urea. The electron-dense cups were missing from the insoluble residues of these detergents. The electron-dense cups and the unique histidine-rich protein appeared to be associated with the RBC skeleton, particularly RBC protein bands 1,2,4.1 and 5. The unique histidine-rich protein binds to the RBC skeleton to form the electron-dense cup. The electron-dense cup produces knobs by forming focal protrusions of the RBC membrane. These protrusions are the specific points of attachment between infected RBC and endothelium.