Endocytic receptors

Intracellular vesicle that carries complexes of endocytic receptors with their ligand cargo internalized from the cell surface. 

Much of our knowledge of the structure and function of endocytic receptors is based on the analysis of the LDL receptor gene family. Member s of this extended gene family can be found in a variety of species ranging from roundworms to insects, to vertebrates. Ten receptors exist in mammalian organisms, all of which share common structural motifs required for receptor-mediated... 

Some proteins are imported into cells from the surrounding medium examples in eukaryotes include low-density lipoprotein (LDL), the iron-carrying protein transferrin, peptide hormones, and circulating proteins destined for degradation. The proteins bind to receptors in invaginations of the membrane called coated pits, which concentrate endocytic receptors in preference to other cell-surface proteins. The pits are coated on their cytosolic side with a lattice of the protein clathrin, which forms closed polyhedral structures (Fig. 27-40). The clathrin lattice grows as more recep-... 

Memmo LM, McKeown-Longo P (1998) The alpha v beta 5 integrin functions as an endocytic receptor for vitronectin. J Cell Sci 111 425-433... 

IF has two binding sites, one for vitamin B12 and one for the ileal receptor cubulin, which recognizes the IF-vitamin B12 complex. Cubulin is synthesized by epithelial cells lining the terminal ileum and other absorptive tissues. It is coexpressed with megalin, a multiligand transmembrane endocytic receptor. The vitamin B12-IF... 

Christensen El, Birn H, Megalin and cubilin multifunctional endocytic receptors, Nature Reviews Molecular Biology, 2002 3 258-68. 

WillnowTE, Receptor-associated protein (RAP) a specialized chaperone for endocytic receptors, Biol Chem, 1998 379 1025-31. 

Verroust PJ, Birn H, Nielsen R, Kozyraki R, Christensen El, The tandem endocytic receptors megalin and cubilin are important proteins in renal pathology. Kidney Int, 2002 62 745-56. 

As far as HDL levels and metabolism are concerned, one result of the LCAT- and transfer protein-catalyzed reactions is the production of a dynamic spectrum of particles with a wide range of sizes and lipid compositions (Chapter 19). Nascent HDL particles contain mostly apo A1 and phospholipids, and undergo modulation and maturation in the circulation. For instance, the unesterified cholesterol incorporated into plasma HDL is converted to cholesteryl esters by LCAT, creating a concentration gradient of cholesterol between HDL and cell membranes, which is required for efficient cholesterol efflux from cells to HDL. In addition, cholesteryl ester transfer protein transfers a significant amount of HDL cholesteryl ester to VLDL, IDL, and LDL for further transport, primarily to the liver. Thus, a substantial fraction of cell-derived cholesterol is delivered as part of HDL indirectly to the liver via hepatic endocytic receptors for IDL and LDL this process is termed reverse cholesterol transport . However, receptor-mediated delivery of HDL cholesterol to cells is fundamentally different from the classic LDL receptor-mediated endocytic pathway, as described in Section 7.3.2. 

The C-type animal lectins include endocytic receptors such as hepatic lectins, macrophage receptors and selectins that mediate the adhesion of leukocytes to... 

Endocytic clathrin-coated vesicle (CCV) formation is a complex process involving a large number of proteins and lipids. The minimum machinery and the hierarchy of the events involved in CCV formation have yet to be dehned. Here we describe an in vitro assay for CCV formation from highly purified rat liver plasma membranes. This rapid and easy assay can be used to quantitatively evaluate the different protein requirements for different endocytic receptors. 

Here we describe a simple in vitro CCV formation assay to quantitatively measure endocytosis of several endocytic receptors at the same time. The assay provides a potential means to analyze the complicated interconnected networks of adaptor and accessory proteins. 

Fig. 1. (A) Diagram of an in vitro assay for CCV formation from PMs. LSP low-speed pellet. LSS low-speed supernatant. HSP high-speed pellet. HSS high-speed supernatant. (B) Temperature-dependent recruitment of endocytic receptors from LSPs to HSPs in the presence of ATP, GTP, and cytosol. Scavenger receptor class B, t5ipe I (SR-BI) is a PM marker that is independent of clathrin-mediated endocytosis. (Figures reproduced from Miwako et al, 2003.)...

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