ACTOMYOSIN MOTOR IN THE MEROZOITE OF THE MALARIA PARASITE, PLASMODIUM-FALCIPARUM - IMPLICATIONS FOR RED-CELL INVASION

The genome of the malaria parasite, Plasmodium falciparum, contains a myosin gene sequence, which bears a close homology to one of the myosi n genes found in another apicomplexan parasite, Toxoplasma gondii, A p olyclonal antibody was generated against an expressed polypeptide of m olecular mass 27,000, based on part of the deduced sequence of this my osin, The antibody reacted with the cognate antigen and with a compone nt of the total parasite protein on immunoblots, but not with vertebra te striated or smooth muscle myosins. It did, however, recognise two c omponents in the cellular protein of Toxoplasma gondii, The antibody w as used to investigate stage-specificity of expression of the myosin ( here designated Pf-myo1) in P. falciparum. The results showed that the protein is synthesised in mature schizonts and is present in merozoit es, but vanishes after the parasite enters the red cell. Pf-myo1 was f ound to be largely, though not entirely, associated with the particula te parasite cell fraction and is thus presumably mainly membrane bound . It was not solubilised by media that would be expected to dissociate actomyosin or myosin filaments, or by non-ionic detergent. Immunofluo rescence revealed that in the merozoite and mature schizont Pf-myo1 is predominantly located around the periphery of the cell. Immune-gold e lectron microscopy also showed the presence of the myosin around almos t the entire parasite periphery, and especially in the region surround ing the apical prominence. Labelling was concentrated under the plasma membrane but was not seen in the apical prominence itself. This sugge sts that Pf-myo1 is associated with the plasma membrane or with the ou ter membrane of the subplasmalemmal cisterna, which forms a lining to the plasma membrane, with a gap at the apical prominence, The results lead to a conjectural model of the invasion mechanism.