The fate of the plasma membrane during endocytosis.

Endocytosis is the only physiological route clearly described so far by which macromolecules and non-permeating micromolecules can gain access to cells. It involves the invagination of the plasma membrane around extracellular material, giving rise to an endocytic vesicle. In most cases, this vesicle will fuse with a lysosome, and a digestive vacuole (secondary lysosome) will be formed in which lysosomal hydrolases are put in contact with the endocytosed material. As a result of this activity, pieces of plasma membrane (henceforth referred to as ‘surface membrane’) are continuously removed from the surface of the cell to constitute the envelope of the endocytic vesicle. The fate of this material is unclear. It has generally been assumed that it is simply incorporated into the membrane of the secondary lysosomes (de Duve & Wattiaux, 1966; Werb & Cohn, 1972). However, lysosomal and plasma membranes differ significantly in antigenic, enzymic and even chemical composition (Henning et al., 1970; Trouet, 1969). Rapid destruction of the specificconstituents of the surface membrane by lysosomal hydrolases has therefore been suggested, and experimental evidence supporting it has been presented (Werb & Cohn, 1972). However, the rate of membrane influx associated with endocytosis was found to be much too high (Steinman e f al., 1976) with respect to the half-life of its constituents (Warren & Glick, 1968) for this hypothesis to be tenable. More complex exchange mechanisms must therefore be sought. We report here experiments in which the fate of the constituents of the surface membrane was investigated during endocytosis. An explanatory model will thereafter be presented and discussed. We conducted experiments with cultured fibroblasts from rat embryo. Cells were incubated in presence of (i) compounds actively endocytosed and (ii) antibodies able to label specifically the surface membrane.