Endocytosis without clathrin coats
Clathrin-dependent endocytosis is not the only
endocytic pathway in animal cells. The use of specific inhibitors of
clathrin-dependent endocytosis, dominant negative forms of Eps15 in particular,
has made it possible to demonstrate the physiological importance of alternative
endocytic pathways. These pathways include caveolae-induced endocytosis. In the
electron microscope, caveolae appear as small invaginations with a typical
"omega–like" shape. Caveolae have a striated coat, one marker of
which is a integral membrane protein, caveolin [45]. Caveolae are
directly involved in the internalization of certain plasma membrane components,
such as GPI-anchored proteins, some toxins, and several envelope viruses [46]. The
detachment of caveolae from the plasma membrane, like that of clathrin-coated
vesicles, is mediated by dynamin [47].
Other, as yet poorly defined, endocytic pathways have
been observed following the inhibition of clathrin-dependent endocytosis, in
cells devoid of caveolae. This is the case for the endocytosis of the
interleukin-2 receptor (IL2-R) [48]. The
constitutive endocytosis of IL2-R has been shown to be coupled to the
partitioning of IL2-R in lipid rafts (dynamic microdomains of the plasma
membrane rich in cholesterol and sphingolipids). Lipid rafts are thought to
play a key role in the endocytosis of other receptors, including the
high-affinity IgE receptor [49].
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