Giardia lamblia: freeze-fracture ultrastructure of the ventral disc plasma membrane.

The close contact of Giardia lamblia trophozoites with mucosal surfaces produces surface indentations of epithelial cells, that progress to areas of microvilli depletion. This interaction is mediated by the lateral crest, a specialized contractile structure in the ventral disc of the parasite. We have analyzed the plasma membrane of the ventral disc using freeze fracture electron microscopy to study the distribution of large integral proteins, and freeze fracture cytochemistry with filipin to reveal sterol-containing sites. A previously undetected structural specialization was found at the lateral crest as a horseshoe-shaped membrane domain characterized by a near absence of intramembrane particles, in sharp contrast to the remainder of the ventral disc plasma membrane. In addition, the lateral crest showed a striking paucity of cholesterol-containing complexes in replicas of trophozoites treated with filipin. These observations demonstrate that the plasma membrane of G. lamblia displays a microheterogeneity in the planar distribution of cholesterol and intramembrane particles. Both membrane components are noticeably less abundant at the outer rim of the ventral disc where contraction takes place.