Clathrin-coated vesicle materials

Both endocytosis of material at the plasma membrane and exocytosis from the Golgi apparatus involve the formation of clathrin-coated pits and vesicles. On the cytosolic side of the membrane these structures have an electron-dense coat consisting mainly of the protein clathrin, the polypeptides of which form a three-legged structure known as a triskelion. The clathrin triskelions assemble into a basket-like convex framework that causes the membrane to invaginate at that point and eventually to pinch off and form a vesicle. In endocytosis these clathrin-coated vesicles migrate into the cell where the clathrin coats are lost before delivering their contents to the lysosomes. 

Figure 41 -15. Two types of endocytosis. An endocytotic vesicle (V) forms as a result of invagination of a portion of the plasma membrane. Fluid-phase endocytosis (A) is random and nondirected. Receptor-mediated endocytosis (B) is selective and occurs in coated pits (CP) lined with the protein clathrin (the fuzzy material). Targeting is provided by receptors (black symbols) specific for a variety of molecules. This results in the formation of a coated vesicle (CV).

As noted above, synaptic vesicles are not typically generated at the level of the TGN. Instead, they are assembled from endocytosed material retrieved from the synaptic plasma membrane. Synaptic vesicle and plasma membrane lipids and proteins are synthesized in the endoplasmic reticulum and modified in the Golgi apparatus, where they are then packaged in secretory vesicles. These synaptic precursors are delivered to the plasma membrane from the cell body by the constitutive secretory pathway. Synaptic vesicle proteins must be retrieved by clathrin-mediated synaptic vesicle endocytosis, a variant of RME with some neuron-specific components. Once the vesicle sheds its clathrin coat, the uncoated vesicle fuses with a... 

In spite of the fact that the driving forces remain speculative, there is now little doubt that, at secretion, lipid droplets are enveloped in apical plasma membrane, with perhaps some contribution from secretory vesicle membrane. Many questions remain, however, regarding the nature and origin of the inner coat material which lies between the triacylglycerol core and the outer bilayer membrane. To what extent is this material derived from the amorphous surface material seen on lipovesicles within the cell (Dylewski et al. 1984 Deeney et al. 1985 Keenan and Dylewski 1985) and the electron-dense coat on the cytoplasmic face of the apical plasma membrane (Franke et al. 1981) Also to be considered is the clathrin-like coat observed on the outer surface of secretory vesicles (Franke et al. 1976 Mather and Keenan 1983), which may contribute a substantial quantity of material to the lipid droplets (Franke and Keenan 1979), if secretory vesicles do indeed contribute to the formation of the milk lipid globule membrane. 

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