Endocytic vesicle

In addition to binding to sialic acid residues of the carbohydrate side chains of cellular proteins that the virus exploits as receptors, hemagglutinin has a second function in the infection of host cells. Viruses, bound to the plasma membrane via their membrane receptors, are taken into the cells by endocytosis. Proton pumps in the membrane of endocytic vesicles that now contain the bound viruses cause an accumulation of protons and a consequent lowering of the pH inside the vesicles. The acidic pH (below pH 6) allows hemagglutinin to fulfill its second role, namely, to act as a membrane fusogen by inducing the fusion of the viral envelope membrane with the membrane of the endosome. This expels the viral RNA into the cytoplasm, where it can begin to replicate. 

Caveolae are invaginations of the plasma membrane. They contain the protein caveolin and are rich in certain phospholipids. Similar to coated pits, they bud off internally forming endocytic vesicles. Caveolae play an important role in the internalization of certain cell surface receptors. 

Clathrin is a protein complex composed of three heavy and three light chains, which assemble in a so-called triskelion. Clathrin is a major constituent of endocytic vesicles. 

Specialized regions of internalization from the plasma membrane, coated with a polyhedral lattice of the protein clathrin. It is in these regions that the first step of the process of endocytosis takes place, with the formation of clathrin-coated endocytic vesicles. 

Figure 11.1 Schematic representation of iron uptake mechanisms, (a) The transferrin-mediated pathway in animals involves receptor-mediated endocytosis of diferric transferrin (Tf), release of iron at the lower pH of the endocytic vesicle and recycling of apoTf. (b) The mechanism in H. influenzae involves extraction of iron from Tf at outer membrane receptors and transport to the inner membrane permease system by a periplasmic ferric binding protein (Fbp). From Baker, 1997. Reproduced by permission of Nature Publishing Group.

The development of amphipathic fluorescent dyes that label endocytic vesicles has permitted the study of endo-cytosis in nerve terminals in real time [25,26], The probe FM1-43 equilibrates between the aqueous phase and the membrane but is not membrane-permeating. The plasmalemma becomes fluorescent (Fig. 10-8). Upon endocytosis, the labeled membrane is internalized. When removed from the extracellular medium, the dye is retained by the endocytic vesicles but lost from the plasmalemma. Endocytic vesicles are transformed into synaptic vesicles containing FM1-43. Importantly, recycled synaptic vesicles lose the probe upon exocytosis. 

Shortly after formation of coated vesicles, the clathrin coat is removed and the vesicles are referred to as endosomes. Endosomes are roughly 300 400 nm in diameter when fully mature. Antibodies to earlier and late endosomes are available from antibodies-online GmbH, Aachen, Germany (http //www.antikoerper-online. de/). Early endosomal antigen 1 (EEA1) is a 162 kDa membrane-bound protein component specific to the early endosomes and is essential for their fusion with early endocytic vesicles for subsequent redistribution of extracellular compounds to... 

The secretory vesicles have recently been discovered by Borregaard and co-workers (Sengelpv, Nielson Borregaard, 1992). These are very difficult to separate from the plasma membrane on density gradients. They possess latent alkaline phosphatase activity (i.e. subcellular fractions must be incubated with detergents such as Triton to release activity) and albumin, whilst the membranes contain CR1, CR3 and the fMet-Leu-Phe receptor. They are endocytic vesicles but can be rapidly translocated to the plasma membrane. 

Van Dyke RW and Root KV [1993] Ethynylestradiol decreases acidification of rat liver endocytic vesicles. Hepatology 18 604-613... 

M. Kirkham, A. Fujita, R. Chadda, S. J. Nixon, T. V. Kurzchalia, D. K. Sharma, R. E. Pagaon, J. F. Hancock, S. Mayor, and R. G. Parton. Ultrastructural identification of uncoated caveolin-independent early endocytic vesicles. J Cell Biol. 168 465-476 (2005). 

The clathrin-coated vesicles bud continuously from the plasma membrane and transport both the plasma membrane and the fluid content of the vesicle into the cell. After entering the cytoplasm, the endocytic vesicle loses its clathrin coat and fuses quickly with other vesicles to form early endosomes. 

Green fluorescent protein-RhoB GFP-RhoB is localized in endocytic vesicles and has been shown to highlight early endosomes, recycling endosomes, and multivesicular bodies, but is absent from lysosomes (123). Because RhoB is toxic when applied for long periods of time, the cells should be analyzed within 24 hours of transient transfection. 

Early endosomes antigen 1, which regulates fusion between endocytic vesicles... 

Rodal SK, Skretting G, Garred O, Vilhardt E, van Deurs B, Sandvig K. Extraction of cholesterol with methyl-beta-cyclodextrin perturbs formation of clathrin-coated endocytic vesicles. Mol Biol Cell 1999 10(4) 961-974. 

Pless DD, Wellner RB. In vitro fusion of endocytic vesicles effects of reagents that alter endosomal pH. J Cell Biochem 1996 62(l) 27-39. 

Pinocytosis is ingestion of small particles and fluid volumes by engulfment and formation of an endocytic vesicle. 

The structure pinches off the plasma membrane and forms an endocytic vesicle, which subsequently loses its clathrin coat. 

Endocytic vesicles fuse with early endosomes, where sorting of the endocy-tosed contents occurs. 

In endocytic vesicle, arrestin dissociates receptor is dephosphorylated and returned to cell surface. 

Electron micrograph of LDL particles (made electron-dense with covalently bound ferritin) bound to coated regions of a human skin fibroblast (97,000 X ). (From R. G. W. Anderson, M. S. Brown, and J. L. Goldstein, Role of the coated and endocytic vesicle in the uptake of receptor-bound low-density lipoprotein in human fibroblasts. Cell 10 351, 1977. Cell Press.)... 

The cellular space, encompassed by the plasma membrane and excluded by organelles and the cytoskeleton, is occupied by the cytoplasm containing macromolecules and small organic and inorganic solutes. Considering that plasmid DNA has to escape from the endocytic vesicles, which are distributed throughout the cytoplasm randomly, it is reasonable to assume that the diffusional mobility of the DNA is an important determinant of the nuclear targeting efficiency of DNA. 

---