Early endosomes

Smad anchor for receptor activation) An intracellular protein Sara which accumulates at early endosomes and plays a key role in TGF- 3 signal transduction through the recruitment of receptor activated R-Smads for phosphorylation by the type ITGF-B receptor. 

Figure 13.9 represents the TEM image of LDH particles and their cellular internalization. As expected, LDH particles are internalized by endocytosis. Figure 13.9(A) shows the cellular uptake process of LDHs after 3h of treatment, and demonstrates a successive entry of LDH by endocytosis first the LDH particles were located around the cell membrane due to their positive charge ( ), then they migrate to the membrane ruffles which are considered as endocytic bodies ( ), finally the coated intracellular vesicles were formed as early endosomes ( ). Figure 13.9(B)...

However, not all proteins proceed directly to their eventual destination. Some proteins relocate from one plasma membrane compartment to another by means of trans-cytosis. Transcytosis involves endocytosis of selected proteins in one membrane compartment, followed by subsequent transport through early endosomes to recycling endosomes and finally translocation to a different membrane compartment, for example from the apical to the basolateral surfaces. Sorting at the TGN and endo-some recycling steps appear to have a primary role in the steady state distribution of proteins in different plasma membrane domains [47], However, selective retention of proteins at the plasma membrane by scaffolding proteins or selective removal may also contribute to normal distributions. Finally, microtubule-motor regulatory mechanisms have been discovered that might explain the specific delivery of membrane proteins to discrete plasma membrane domains [48]. 

Ligand (LDL, transferrin, etc.) Recycling receptor Retrograde vesicle Early endosome... 

FIGURE 2 3-7 Schematic diagram of the synaptic vesicle cycle. Neurotransmitter-filled vesicles held in the reserve pool are trafficked to a readily releasable pool where they are docked, primed and fused with the plasmalemma at the synaptic cleft. Also depicted is the clathrin-mediated endocy-tosis of the fused vesicles, which is followed by their uncoating and recycling via early endosomal fusion and budding of vesicles. This returns the vesicles to the reserve pool. Some of the phosphoproteins which regulate these steps are shown. For a more detailed description of this process and the phosphoproteins involved the reader is directed to the excellent text by Cowen et al. [67]. 

Synncs, M., Prydz, K., Lovdal, T., Brech, A. and Berg, T. (1999). Fluid phase endocytosis and galactosyl receptor-mediated endocytosis employ different early endosomes, Biochim. Biophys. Acta-Biomembranes, 1421, 317-328. 

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... 

In endocytosis, vesicles are formed at the plasma membrane and then transported to an endosome. (More precisely, endosomes should at least be classified into early endosomes and late endosomes, but this fact is ignored here.) The endocytic pathway also includes the following routes from the endosome to the lysosome, from the endosome to the plasma... 

Mu, F. T., et al., EEAl, an early endosome-associated protein. EEAl is a conserved alpha-helical peripheral membrane protein flanked by cysteine fingers and contains a calmodulinbinding IQ motif. J Biol Chcm, 1995, 270(22), 13503-11. 

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. 

Early endosomes are the main sorting station in the endocytic pathway. In their acidic interior (pH 5.9-6.0), the receptor and its ligand can be released. The receptor may be recycled to the surface by vesicles that fuse with the plasma membrane. Material that cannot escape from the early endosomes is further transported via multivesicular bodies to late endosomes and digesting lysosomes that contain a broad spectrum of peptidases and hydrolases in an acidic surrounding [for reviews on endocytosis see Refs. (10-12), for review on clathrin uptake see Refs. (9,13)]. 

Early endosomes 16°C Block transport from early 76,88... 

Either fluorescent EGF or its fluorescent receptor (GFP-EGFR) can be used to track the clathrin pathway (110). Following internalization, EGF is mainly targeted to early endosomes, then to lysosomes, and following this is subsequently degraded. After a 10-minute incubation period with EGF, the polypeptide hormone accumulates in early endosomes, and after 60 to 90 minutes, EGF... 

Several aspects of intracellular trafficking should be kept in mind in the intracellular trafficking section. The first is the dependence of acidification of endosomes on the uptake of liposomes. This aspect is sometimes discussed when analyzing clathrin uptake. However, several other pathways are also in need of acidic compartments as a destination of uptake so, we list this factor as an individual aspect. Other aspects of intracellular trafficking that are of interest are the transport from early endosomes to late endosomes, the dependence of actin filaments and dynamin, and/or microtubules. Furthermore, the energy dependence of liposome uptake is discussed. 

As described above (section Clathrin-Mediated Uptake ), several ligands for clathrin-mediated endocytosis (see section Clathrin-Mediated Endocytosis EGF, Tfn, LDL) can be used to highlight early endosomes or lysosomes, depending on different incubation times. 

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... 

Microtubules are used to cover relatively long distances in the cell. Therefore, the short distance between the cell surface and the early endosomes does not require microtubules (94,136). They are involved in the later steps in endocytosis early endosomes accumulate endocytic material for about 10 minutes and then generate transport vesicles (0.5 pm in diameter), which will be taken to the cell center with a relatively high traveling speed of Ipm/sec (with velocities of up to 2.5pm/sec) (95,137). These transport vesicle move on microtubule-tracks to the cell center (and to the ly sosome) (94). When performing live cell imaging studies, it should be kept in mind that microtubules are extremely sensitive to ultraviolet light, which causes their polymerization. 

Gruenberg J, Griffiths G, Howell KE. Characterization of the early endosome and putative endocytic carrier vesicles in vivo and with an assay of vesicle fusion in vitro. J Cell Biol 1989 108(4) 1301-1316. 

Figure 1 The mode of action for bacterial AB-type exotoxins. AB-toxins are enzymes that modify specific substrate molecules in the cytosol of eukaryotic cells. Besides the enzyme domain (A-domain), AB-toxins have a binding/translocation domain (B-domain) that specifically interacts with a cell-surface receptor and facilitates internalization of the toxin into cellular transport vesicles, such as endosomes. In many cases, the B-domain mediates translocation of the A-domain into the cytosol by pore formation in cellular membranes. By following receptor-mediated endocytosis, AB-type toxins exploit normal vesicle traffic pathways into cells. One type of toxin escapes from early acidified endosomes (EE) into the cytosol, thus they are referred to as short-trip-toxins . In contrast, the long-trip-toxins take a retrograde route from early endosomes (EE) through late endosomes (LE), trans-Golgi network (TGN), and Golgi apparatus into the endoplasmic reticulum (ER) from where the A-domains translocate into the cytosol to modify specific substrates.

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