Maurer's cleft organization in the cytoplasm of Plasmodium falciparum-infected erythrocytes: new insights from three-dimensional reconstruction of serial ultrathin sections

Abstract

Maurer's clefts are single-membrane-limited structures in the cytoplasm of erythrocytes infected with the human malarial parasite Plasmodium falciparum. The currently accepted model suggests that Maurer's clefts act as an intermediate compartment in protein transport processes from the parasite across the cytoplasm of the host cell to the erythrocyte surface, by receiving and delivering protein cargo packed in vesicles. This model is mainly based on two observations. Firstly, single-section electron micrographs have shown, within the cytoplasm of infected erythrocytes, stacks of long slender membranes in close vicinity to round membrane profiles considered to be vesicles. Secondly, proteins that are transported from the parasite to the erythrocyte surface as well as proteins facilitating the budding of vesicles have been found in association with Maurer's clefts. Verification of this model would be greatly assisted by a better understanding of the morphology, dimensions and origin of the Maurer's clefts. Here, we have generated and analyzed three-dimensional reconstructions of serial ultrathin sections covering segments of P. falciparum-infected erythrocytes of more than 1 microm thickness. Our results indicate that Maurer's clefts are heterogeneous in structure and size. We have found Maurer's clefts consisting of a single disk-shaped cisternae localized beneath the plasma membrane. In other examples, Maurer' clefts formed an extended membranous network that bridged most of the distance between the parasite and the plasma membrane of the host erythrocyte. Maurer's cleft membrane networks were composed of both branched membrane tubules and stacked disk-shaped membrane cisternae that eventually formed whorls. Maurer's clefts were visible in other cells as a loose membrane reticulum composed of scattered tubular and disk-shaped membrane profiles. We have not seen clearly discernable isolated vesicles in the analyzed erythrocyte segments suggesting that the current view of how proteins are transported within the Plasmodium-infected erythrocyte may need reconsideration.