mixing of contents of endosomes and lysosomes. Early endosomes
lipid flow in the secretory pathway; it is the site where protein and lipid
derive from endocytic vesicles (clathrin-coated vesicles and caveolae).
glycosylation occurs; and it provides membrane platforms to which
Once internalized, endocytic vesicles shed their coat of adaptin and
signalling and sorting proteins bind.
clathrin, and fuse to form an early endosome, where the receptor
Ultrastructurally, the Golgi apparatus (Fig. 1.5A) displays a contin-
molecules release their bound ligands. Membrane and receptors from
uous ribbon-like structure consisting of a stack of several flattened
the early endosomes can be recycled to the cell surface as exocytic
membranous cisternae, together with clusters of vesicles surrounding
vesicles.
its surfaces. Cisternae differ in enzymatic content and activity. Small
Clathrin-dependent endocytosis occurs at specialized patches of
transport vesicles from the rough endoplasmic reticulum are received
plasma membrane called coated pits; this mechanism is also used to