A Unique Histidine-rich Polypeptide from the Malaria Parasite, Plasmodium lophurae

Abstract

Abstract A single basic polypeptide was found to be the main chemical constituent of the characteristic cytoplasmic granules of Plasmodium lophurae. When sonicated suspensions of parasites (freed from host erythrocytes) were centrifuged through 1.7 m sucrose, the resulting pellet on examination by electron microscopy showed only a mixture of the granules and contaminating hemozoin (malarial pigment). The granules could be extracted completely with 0.9 n acetic acid either after removal of hemozoin with 0.1 n NaOH or directly from the sucrose pellet. Only hemozoin was left as residue in the latter case. The acid extracts from both procedures subjected to electrophoresis gave a single band on polyacrylamide gels. In 6 m guanidine hydrochloride, the polypeptide eluted as a single peak from a calibrated Sephadex G-200 column at an estimated molecular weight of 35,000 to 40,000. The soluble polypeptide reaggregated into granules on dialysis in slightly alkaline buffers. The amino acid analysis of the polypeptide showed a most unusual composition with five amino acids accounting for 95% of the total mole ratio: 73% histidine, 7.5% proline, 7% alanine, 6% glutamic acid, and 2.1% aspartic acid. The polypeptide accounts for about 11% of the dry weight of multinucleate and 3% of uninucleate malaria trophozoites.