Endocytosis

It has been further shown that a majority (approximately 60jt) of the extractable Qa (about 70 of the cellular gallium) from tumor and liver cells of the rat is associated with two macromolecular fractions of molar weight 1-1.2 x 105 Daltons and 4-5 X 10 Daltons (64) The 1-1.2 x 1o5 D band is found in both liver and tumor cells, whereas the 4-5 x 10 D band is found 

ACS Symposium Series American Chemical Society Washington, DC, 1980. 

Lactoferrin, with a molecular weight of 8.5-9.0 x 10 and a structure similar to transferrin (112) has been suggested as an alternative intracellular gallium-binding agent (113). 

Lactoferrin binds iron with a greater affinity than transferrin (114). and is found in tissues and secretions (especially milk) which localize galllum-67 (115.116.117.118.119). It was proposed that Ga labeled to transferrin and other plasma proteins Is transferred to cellular lactoferrin due to the latter s greater chelating ability (113-120). Such a transfer of gallium-67 has been demonstrated in vitro (121), and increased concentrations of the protein has been found in tumors (122.123.124). 

Perhaps the most exciting aspect of this proposed mechanism is the manner in which it correlates with recently-isolated lon-blndlng molecules called slderophores (129). The primary function of lactoferrin is to diminish the amount of extracellular free iron and thereby inhibit bacterial growth (130). Lactoferrin deposited by polymorphonuclear leukocytes is attached to the surface of monocytes and macrophages in inflammatory responses (130). Slderophores are synthesized by bacterial cells to sequester iron needed for growth, and 

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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. 

This fusogenic activity of influenza hemagglutinin is frequently exploited in the laboratory. If, for example, the virus is bound to cells at a temperature too low for endocytosis and then the pH of the external medium is lowered, the hemagglutinin causes direct fusion of the viral envelope with the plasma membrane infection is achieved without endocytosis. Similarly, artificial vesicles with hemagglutinin in their membrane and other molecules in their lumen can be caused to fuse with cells by first allowing the vesicles to bind to the plasma membrane via the hemagglutinin and then lowering the pH of the medium. In this way the contents of the vesicles are delivered to the recipient cell s cytoplasm. 

FIGURE 25.39 Endocytosis and degradation of lipoprotein particles. (ACAT is acyl-CoA cholesterol acyltransferase.)... 

They encapsulated poly(MA-CDA) into mannan-coated liposomes and evaluated superoxide production from mouse macrophages. The activity was three- to five-fold high compared with uncapsulated poly(MA-CDA) itself [5,11], suggesting that an increased incorporation of the polymer by the receptor-mediated endocytosis mediated the higher biological activity.. 

C1C-3, -4 and -5 form the second branch of the CLC gene family. These proteins are 80% identical, and with the exception of C1C-5, which is most highly expressed in kidney and intestine, show a broad expression pattern. C1C-3 to C1C-5 reside in intracellular membranes of the endocytotic pathway [4]. Disruption of C1C-5 leads to a defect in endocytosis in mouse... 

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. 

A process in which a substance gains entry into a cell. Endocytic mechanisms are crucial for a variety of cellular functions such as the uptake of nutrients, regulation of cell surface expression of receptors, maintenance of cell polarity, and more. Receptor-mediated endocytosis via clathrin-coated pits is the most studied endocytic process, which is important for regulation of the time and magnitude of signals generated by a variety of cell-surface receptors. 

Marmor MD, Yarden Y (2004) Role of protein ubiquity la-tion in regulating endocytosis of receptor tyrosine kinases. Oncogene 23 2057-2070... 

Internalization is an agonist-induced endocytosis of membranous receptors which occurs in seconds to minutes. It involves the formation of receptor containing... 

Biochemical characterization of clathrin-coated vesicles revealed that their major coat components are clathrin and various types of adaptor complexes. Clathrin assembles in triskelions that consist of three heavy chains of approximately 190 kDa and three light chains of 30 40 kDa. Four types of adaptor complexes have been identified to date, AP-1, AP-2, AP-3 and AP-4 (AP for adaptor protein). Whereas AP-1, AP-3 and AP-4 mediate sorting events at the TGN and/or endosomes, AP-2 is involved in endocytosis at the plasma membrane. Each adaptor complex is a hetero-tetrameric protein complex, and the term adaptin was extended to all subunits of these complexes. One complex is composed of two large adaptins (one each of y/a/S/s and [31-4, respectively, 90-130 kDa), one medium adaptin (pi -4, <50 kDa), and one small adaptin (ol-4, <20 kDa). In contrast to AP-1, AP-2 and AP-3, which interact directly with clathrin and are part of the clathrin-coated vesicles, AP-4 seems to be involved in budding of a certain type of non-clathrin-coated vesicles at the TGN. 

Mucolipin, also known as mucolipin 1 or mucolipidin (encoded by the MCOLN1 gene), is a TRP channel-related membrane protein, most probably residing in intracellular membranes. Is defective in mucolipidosis type IV disease, a developmental neurodegenerative disorder characterized by lysosomal storage disorder and abnormal endocytosis of lipids. The fimction of mucolipin is unknown. 

NHE5. The distribution of this isoform is distinct, being in neuronal-rich areas of the central nervous system. Low levels have also been found in testis, spleen and skeletal muscle. Like the preceding isoforms, NHE5 is found in the plasma membrane and is internalised by clathrin-associated endocytosis into recycling endosomes. The normal role of NHE5 is unknown but its malfunction is speculated to contribute to the development of neurodegenerative disease. 

Sorkin A, Zastrow M Von (2002) Signal transduction and endocytosis close encounters of many kinds. Nat Rev Mol Cell Biol 3 600-614... 

Ubiquitin/Proteasome. Figure 2 Functional consequences of ubiquitin linkage. Substrates (blue bars) are linked via lysine residues (K) to ubiquitin or ubiquitin chains, (a) Attachment of chains connected via Lysines in position 48 of ubiquitin (K48) targets substrates for proteasomal degradation. In contrast modification of one (b) or multiple (c) lysines by a single ubiquitin molecule mediates novel protein interactions or initiates endocytosis. Conjugation of K63-linked polyubiquitin (d) alters protein function and can also serve as a signal for endocytosis. 

Mukhopadhyay D, Riezman H (2007) Proteasome-independent functions of ubiquitin in endocytosis and signaling. Science 315 201-205... 

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