Several weak signals in the cytosolic and transmembrane domains of the interleukin‐2‐receptor β chain allow for its efficient endocytosis
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Abstract:
Plasma membrane receptors are retrieved continually from the cell surface by endocytosis and transported to intracellular organelles. They are internalized at various rates depending on their ability to interact with endocytic structures of the plasma membrane. The interleukin-2-receptor beta chain is endocytosed constitutively and efficiently. Here we show that different motifs in its cytosolic tail promote entry in an additive way, each of them acting as a weak internalization signal. The transmembrane domain of beta also participates in endocytosis. In conclusion, several weak endocytic determinants can be responsible for the rapid internalization of a membrane protein.Keywords:
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Cell surface receptor
Bulk endocytosis
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ABSTRACT During the last decade the term ‘endocytosis’ has become virtually synonymous with the activity of clathrin-coated vesicles. These vesicles, which are derived from cell surface clathrin-coated pits, are transport vehicles responsible for the transfer of plasma membrane receptors and their ligands, between the first two stations of the endocytic pathway: namely, the plasma membrane and early endosomes (Goldstein et al., 1985; van Deurs et al., 1989; Griffiths and Gruenberg, 1991). Despite the irrefutable evidence that clathrin-coated vesicles mediate endocytosis, their contribution to the total endocytic activity of the cell and the composition of the membrane they internalise remains controversial. Here we discuss: (1) the evidence that non-clathrin-mediated endocytic mechanisms operate alongside the clathrin-mediated pathway; (2) the evidence that endocytosis occurs for surface molecules that are not enriched in clathrin-coated pits and; (3) the sorting activities of cell surface clathrincoated pits and the notion that plasma membrane proteins that show particularly slow rates of uptake are actively excluded from the endocytic pathway.
Clathrin adaptor proteins
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Cell membrane
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Bulk endocytosis
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Bulk endocytosis
Amphiphysin
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Key features of clathrin-associated endocytosis are outlined, and current evidence in favor of clathrin-independent endocytosis and of its structural counterpart(s) is reviewed. We next summarize our recent observations on clathrin-independent endocytosis in primary cultures of rat foetal fibroblasts, using two reversible inhibitors of the formation of endocytic clathrin-coated pits, Severe inhibition of clathrin polymerization at the plasma membrane slows down receptor-mediated endocytosis of transferrin by ten-fold, without affecting bulk-flow endocytosis of fluid and membrane. Furthermore, the size of primary endocytic vesicles, identified by ultrastructural cytochemistry; is the same in control and treated cells. Two interpretations are offered. The most provocative one proposes that clathrin plays no role in the formation of primary endocytic vesicles, and is only required to concentrate receptors in endocytic pits, accelerating thereby internalization of ligand-receptor complexes. In the second interpretation, inhibition of clathrin polymerization unmasks an accessory molecular machinery, which is not operating under control conditions. In both cases, another endocytic molecular machinery is required and remains to be identified.
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A stable HeLa cell line expressing a dynamin mutant, dynts, exhibits a temperature-sensitive defect in endocytic clathrin-coated vesicle formation. Dynts carries a point mutation, G273D, corresponding to the Drosophila shibirets1 allele. The ts-defect in receptor-mediated endocytosis shows a rapid onset (< 5 min) and is readily reversible. At the nonpermissive temperature (38 degrees C) HRP uptake is only partially inhibited. Moreover, when cells are held at the nonpermissive temperature, fluid phase uptake fully recovers to wild-type levels within 30 min, while receptor-mediated endocytosis remains inhibited. The residual HRP uptake early after shift to the nonpermissive temperature and the induced HRP uptake that occurs after recovery are insensitive to cytosol acidification under conditions that potently inhibit receptor-mediated endocytosis of Tfn. Together, these results suggest that a dynamin- and clathrin-independent mechanism contributes to the total constitutive pinocytosis in HeLa cells and that dynts cells rapidly and completely compensate for the loss of clathrin-dependent endocytosis by inducing an alternate endocytic pathway.
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Endocytosis in animal cells has been heavily documented. Both fluid‐phase and receptor‐mediated modes of uptake have been frequently studied, and the endocytic pathway is well defined. This contrasts markedly with the situation in plants where our knowledge of this process is still rudimentary. Partly responsible for this situation has been the view, widely held among plant physiologists, that because of turgor, endocytosis cannot occur in plant cells. As discussed below, the case against endocytosis is no longer water‐tight. Endocytosis is a fact in protoplasts. It can be demonstrated with electron‐dense tracers as well as with membrane impermeant dye Lucifer Yellow CH. The latter has also been used with success on both suspension‐cultured and tissue cells of higher plants, suggesting that fluid‐phase endocytosis is also a feature of cells with walls. Through the application of fluorescently labelled elicitor molecules, which specifically bind to the cell surface of suspension‐cultured cells, it has also been possible to provide convincing evidence for the operation of receptor‐mediated endocytosis in plants. A number of studies on protoplasts and cells clearly indicate that endocytosis in plants can be mediated by coated pits in the plasma membrane. At least one of the organelles that lie on the endocytic pathway in plants has a structurally similar counterpart in animal cells: the multivesicular body. The first recipient of internalized molecules is the partially coated reticulum, although its relationship to the Golgi apparatus and Golgi function remain to be clarified. The final target for endocytosis in plants appears to be the vacuole.
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Endocytosis of glycosylphosphatidyl inositol (GPI)-anchored proteins (GPI-APs) and the fluid phase takes place primarily through a dynamin- and clathrin-independent, Cdc42-regulated pinocytic mechanism. This mechanism is mediated by primary carriers called clathrin-independent carriers (CLICs), which fuse to form tubular early endocytic compartments called GPI-AP enriched endosomal compartments (GEECs). Here, we show that reduction in activity or levels of ARF1 specifically inhibits GPI-AP and fluid-phase endocytosis without affecting other clathrin-dependent or independent endocytic pathways. ARF1 is activated at distinct sites on the plasma membrane, and by the recruitment of RhoGAP domain-containing protein, ARHGAP10, to the plasma membrane, modulates cell-surface Cdc42 dynamics. This results in the coupling of ARF1 and Cdc42 activity to regulate endocytosis at the plasma membrane. These findings provide a molecular basis for a crosstalk of endocytosis with secretion by the sharing of a key regulator of secretory traffic, ARF1. PMID: 18084285
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Pinocytosis
Bulk endocytosis
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Internalization
Transferrin receptor
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Pinocytosis
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