Cryofracture electron microscopy of the ookinete pellicle of Plasmodium gallinaceum reveals the existence of novel pores in the alveolar membranes

The malaria parasite invades the midgut tissue of its mosquito host as a mo tile form called the ookinete. We have examined the pellicle of the ookinet e of Plasmodium gallinaceum by freeze-fracture and quick-freeze, deep-etch electron microscopy. The general organization is analogous to that of invas ive stages of other members of Apicomplexa. The pellicle is composed of thr ee membranes: the plasma membrane, and the two linked intermediate and inne r membranes, which in the ookinete form one flattened vacuole that is locat ed beneath the plasma membrane. The edges of this vacuole form a longitudin al suture. Beneath the vacuole is found an array of microtubules that are c onnected to the inner membrane by intramembranous particles. During freeze- fracture, the membranes can split along their hydrophobic planes, thus yiel ding six fracture faces, each of which displays a characteristic pattern of intramembranous particles. Additionally, we find that the ookinete pellicl e differs from all other apicomplexan motile stages by the presence of larg e pores. These pores are of unknown function, but clearly might constitute a novel pathway for the transport of molecules to and from the cortex, whic h is independent of the well-described route through the apical micronemal/ rhoptry complex. The pores may be the route by which motor proteins or othe r nonmicronemal surface proteins are trafficked, such as P25/P28 and SOAP, some of which are implicated in transmission blocking immunity. (C) 2001 Ac ademic Press.