N-terminal amino acid sequence of the histidine-rich protein from Plasmodium lophurae

We have determined the N-terminal amino acid sequence of the first 25 amino acids of the histidine-rich protein (HisRP) isolated from granules of the avian malaria parasite Plasmodium lophurae. The protein was purified from cytoplasmic granules and shown to be 65.2 mol % histidine, close to the previously described value of 73 mol % histidine (Kilejian (1974) J. Biol. Chem. 249, 4650–4655). Ten of the first 25 residues were histidine, five of which formed the sequence His-His-His-His-His (positions 14–18). Also notable was the presence of eight acidic residues within the N-terminal 25 residues. HisRP contained no detectable carbohydrate. When the HisRP was biosynthetically labeled in cultured infected erythrocytes, incorporation of [ 3H]His greatly exceeded [ 3H]Ile. Labeled HisRP was not solubilized with 1% w/v Triton X-100 but could be solubilized with ⩾ 1% w/v sodium dodecyl sulfate. On sodium dodecyl sulfate-polyacrylamide gel electrophoresis the [ 3H]His labeled protein migrated as a doublet ( M r 53 000 and 50 000). Only one of these bands ( M r 53 000) comigrated with the Coomassie Blue stained protein isolated by the acid-extraction procedure from purified granules. The amino acid composition of HisRP and presence of five contiguous histidine residues in the sequence studied here suggests that other sequences of several contiguous histidine residues must exist in this molecule.