Abstract
Membranes of erythrocytes infected with the human malaria parasite Plasmodium falciparum develop protrusions called knobs. These structures are essential for the survival of the parasite in the host, and their induction requires the synthesis of the knob protein by the parasite. We describe the isolation of a cDNA clone encoding the amino-terminal half of the knob protein. A cDNA library was constructed from RNA prepared from ring stages of a P. falciparum isolate that has retained its ability to induce knobs (knob+ phenotype). A synthetic oligonucleotide probe encoding polyhistidine was used to isolate the cDNA clone, which encodes the amino-terminal half of a polypeptide with all the known attributes of the knob protein. The gene is not transcribed in variants that do not synthesize the knob protein and thereby cannot induce knobs (knob- phenotype). The apparent lack of transcription in knob- variants is due to different mechanisms: although the gene is present in one knob- isolate, it has been deleted in a cloned knob- variant. The primary structure of the polypeptide deduced from a partial sequence of the cDNA is distinctly different from other malarial histidine-rich polypeptides. The amino-terminal sequence shows the characteristic features of a signal peptide. This is followed by a histidine-rich domain and a subsequent region which contains one histidine. Peptide map analysis of the knob protein is consistent with the structural features deduced from the sequence analysis of the cDNA.