Lumen

Figure 2 Targeting/capture of m-exocytotic and s-exocytotic vesicles to/by specialized cell membrane domains. A polarized epithelial cell is depicted, but many of the same features are found in other cell types (neurons, macrophages, lymphocytes, yeast). A, Transfer of substances across an epithelial ceil (endothelium, kidney tubule) via a transcellular path from apical (endocytosis) to basal (s-exocytosis) domains; B, uptake of extracellular material (endocytosis) for transfer to luminal space (s-exocytosis) from basal to luminal domains (IgA, IgM across enterocyte); C, m-exocytosis of integral membrane protein (uncharacterlzed) to tight junction between lateral domains of abutting epithelial cell membranes; D, m-exocytosis of adhesion integral membrane proteins (CAMs, integrlns) to form spot and belt desmosomes and hemidesmosomes at lateral and basal membrane domains; E, m-exocytosis of integral membrane proteins (connexin) to communicating (electrotonic) gap junctions in the lateral domain of abutting cells; F, m-exocytosis of receptors to basal membrane (binding to fibronectins, glycosaminoproteoglycans, intermediate filaments). Specialized cell junctions are transient in that they can be withdrawn and restored rapidly under appropriate conditions. The bimolecular unit membranes are drawn as a single line. Note that s- and m-endocytosis are one and the same process that here has been dissociated for didactic purposes to emphasize the overlapping though different end-points.

Figure 2 Targeting/capture of m-exocytotic and s-exocytotic vesicles to/by specialized cell membrane domains. A polarized epithelial cell is depicted, but many of the same features are found in other cell types (neurons, macrophages, lymphocytes, yeast). A, Transfer of substances across an epithelial ceil (endothelium, kidney tubule) via a transcellular path from apical (endocytosis) to basal (s-exocytosis) domains; B, uptake of extracellular material (endocytosis) for transfer to luminal space (s-exocytosis) from basal to luminal domains (IgA, IgM across enterocyte); C, m-exocytosis of integral membrane protein (uncharacterlzed) to tight junction between lateral domains of abutting epithelial cell membranes; D, m-exocytosis of adhesion integral membrane proteins (CAMs, integrlns) to form spot and belt desmosomes and hemidesmosomes at lateral and basal membrane domains; E, m-exocytosis of integral membrane proteins (connexin) to communicating (electrotonic) gap junctions in the lateral domain of abutting cells; F, m-exocytosis of receptors to basal membrane (binding to fibronectins, glycosaminoproteoglycans, intermediate filaments). Specialized cell junctions are transient in that they can be withdrawn and restored rapidly under appropriate conditions. The bimolecular unit membranes are drawn as a single line. Note that s- and m-endocytosis are one and the same process that here has been dissociated for didactic purposes to emphasize the overlapping though different end-points.

exTHACeLiULAR SPACE

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Figure 3 Endocytosis-exocytosis coupling, membrane conservation and receptor recycling. Integral membrane proteins such as receptors delivered to the plasma membrane by m-exocytosis (Figure 1) bind specific ligands, and the complexes then aggregate into clathrin triskelions-coated pits that are internalized by endocytosis of vesicles linked to and pulled off the plasmalemma by actin filaments. After internalization, the triskelion cage depolymerizes and the vesicle connects to microtubules via cytoplasmic dynein motor proteins that move it along retrogradely. The vesicles join and fuse with lysosomes shed off the trans-Golgi network and propelled anterogradely by kinesin motor proteins, also along microtubules. The fused vesicles become acidified by an ATP-driven proton pump, the receptor-ligand complex dissociates and the lysosome-vesicle organelle (secondary lysosome) reseparates into lysosome and vesicle moieties. The freed ligand, sequestered within the secondary lysosome, is degraded by acid hydrolases, while the receptors sequestered within the regenerated m-exocytotic vesicle membrane are translocated back to the plasmalemma for recycling. TGN, trans-Golgi network complex; H", proton; enz, lysosomal enzymes; triskelions, clathrin trimers. Note that s- and m-endocytosis are one and the same process that here has been dissociated for didactic purposes to emphasize the overlapping though different end-points.

rn-Exo«yt«i5 oi recycled receptor*

Figure 3 Endocytosis-exocytosis coupling, membrane conservation and receptor recycling. Integral membrane proteins such as receptors delivered to the plasma membrane by m-exocytosis (Figure 1) bind specific ligands, and the complexes then aggregate into clathrin triskelions-coated pits that are internalized by endocytosis of vesicles linked to and pulled off the plasmalemma by actin filaments. After internalization, the triskelion cage depolymerizes and the vesicle connects to microtubules via cytoplasmic dynein motor proteins that move it along retrogradely. The vesicles join and fuse with lysosomes shed off the trans-Golgi network and propelled anterogradely by kinesin motor proteins, also along microtubules. The fused vesicles become acidified by an ATP-driven proton pump, the receptor-ligand complex dissociates and the lysosome-vesicle organelle (secondary lysosome) reseparates into lysosome and vesicle moieties. The freed ligand, sequestered within the secondary lysosome, is degraded by acid hydrolases, while the receptors sequestered within the regenerated m-exocytotic vesicle membrane are translocated back to the plasmalemma for recycling. TGN, trans-Golgi network complex; H", proton; enz, lysosomal enzymes; triskelions, clathrin trimers. Note that s- and m-endocytosis are one and the same process that here has been dissociated for didactic purposes to emphasize the overlapping though different end-points.

breathe life into the organism and assure survival of its DNA.

See also: Adhesion molecules; Antigens, cell surface; Atopic allergy; Capping, clustering, membrane microdomains and all surface dynamics; CD antigens; Cell-mediated immunity; Complement receptors; Degranulation; Fc receptors; Hypersensitivity reactions; Immunoglobulin, cell surface; Mast cells; Phagocytosis.

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