Transmembrane receptor Structure

Frizzled (Fz) proteins comprise a family of seven-pass transmembrane receptors with a cysteine-rich extracellular domain. As a class, Fz proteins are structurally related to the superfamily ofheterotrimeric G-protein coupled receptors (GPCRs). Diere are 4 Fz genes in Drosophila and 10 in humans, with close orthologs... 

The human oxytocin receptor gene was isolated and characterised in 1994 [122], heralding the development of modern cloned receptor screening. The oxytocin receptor belongs to the Family A series of G-protein coupled 7-transmembrane receptors (GPCRs). A schematic representation of the generic structure of 7TM receptors is shown in Figure 7.3. 

The structural analysis of membrane-associated peptides comprises two steps (a) the elucidation of the three-dimensional fold of the peptide and (b) the determination of the membrane-peptide interface. We will use our results gained for the 36 amino acid residue neuropeptide Y (NPY) [83] to demonstrate the approaches that can be used. NPY regulates important pharmacological functions such as blood pressure, food intake or memory retention and hence has been subject of many investigations (for a review see Ref. [84]). It targets the so-called Y receptors that belong to the class of seven transmembrane receptors coupled to G-proteins (GPCRs). 

Clathrin-mediated endocytosis involves the internalization of transmembrane receptor-ligand complexes stimulating the formation of a coated pit that eventually buds off the membrane to form an intracellular endocy-totic vesicle. This process is dependent on the protein clathrin that is recruited to the membrane and forms a cage-like structure around the forming pit. Internalization via clathrin-dependent pathway allows the uptake of particles approximately 120nm in size (63-65). Once internalized, the clathrin coating disassociates from the endosome to be recycled and to allow the endosome to fuse with an intracellular compartment, usually a... 

Over the last decade, a substantial amoimt of information about properties and spatial structures of different representatives of the TGF- superfamily, both free and in complexes with ectodomains of their transmembrane receptors, have been accumulated. Consequently, a number of important steps towards understanding initiation and actuation of TGF-b signahng, as well as the biological response to the signal transduction and interplay with other signaling pathways have been made [2,12,18,46-50]. 

Gudermann, T., Nurnberg, B., and Schultz, G. Receptors and G proteins as primary components of transmembrane signal transduction. Part 1. G-protein-coupled receptors structure and function./. Mol. Med. 1995, 73, 51-63. 

Receptors are macromolecules of peptidic structure (a three-dimensionally arranged sequence of amino acids) that are predominantly located within cell membranes. One common structural form is the so-called seven-transmembrane receptor, which consists of seven domains located within the membrane plus an extracellular (top of Fig. 4.3) and an intracellular (bottom of Fig. 4.3) part. Within the receptor molecule there is also a specific binding pocket (not shown in the figure) for a message molecule (a neurotransmitter or any other ligand) and this part of the three-dimensionally arranged receptor molecule is named the binding site. 

In comparison to signaling pathways which utilize transmembrane receptors (see chapter 5, 8,11), signahng via nuclear receptors is of relatively simple structure. The pathways lead directly, with only a few participating protein components, from the extracellular space to the level of the DNA in the nucleus. Most important protein components of the signal pathway are known and well characterized. Nevertheless, we understand very little of the mechanism by which the activated receptors lead to a transcription initiation. This is due to the extreme complexity of transcription initiation in eucaryotes (see 1.2). Both the variety of proteins involved in the formation of a competent initiation complex, as well as the influence of chromatin structure, make it difficult to elucidate the exact function of nuclear receptors in transcription initiation. 

Transmembrane receptors are integral membrane proteins, i.e., they possess a structural portion that spans the membrane. An extracellular domain, a transmembrane domain and an intracellular or cytosolic domain can be differentiated within the structure (Fig. 5.2a). 

Transmembrane receptors may show homotrophic composition (identical subunits) or heterotrophic composition (different subunits Fig. 5.2b), so that the extracellular domain may be made up of several identical or different structural elements. 

Fig. 5.2. Structural principles of transmembrane receptors, a) Representation of the most important functional domains of transmembrane receptors, b) Examples of subunit structures. Transmembrane receptors can exist in a monomeric form (1), dimeric form (2) and as higher oligomers (3,4). Further subunits may associate at the extracellular and cytosohc domains, via disulfide bridges (3) or via non-covalent interactions (4). c) Examples of structures of the transmembrane domains of receptors. The transmembrane domain may be composed of an a-hehx (1) or several a-helices linked by loops at the cytosolic and extracellular side (2). The 7-helix transmembrane receptors are a frequently occurring receptor type (see 5.3). Several subunits of a transmembrane protein may associate into an ohgomeric structure (3), as is the case for voltage-controUed ion channels (e.g., K channel) or for receptors with intrinsic ion channel function (see Chapter 17).

The transmembrane receptors span the ca. 5 mn thick phospholipid bilayer of the cell membrane with structural portions known as transmembrane elements. The inner of a phosphohpid layer is hydrophobic and, correspondingly, the surface of the structural elements that come into contact with the iimer of the phospholipid double layer also has hydrophobic character. 

High resolution structural information about the transmembrane elements of membrane receptors is not currently available, since it is not yet possible to obtain transmembrane receptors in crystalline form for structural analysis. Due to the hydrophobic nature of the transmembrane elements, crystallization is very difficult. 

At present, it is generally assumed that transmembrane receptors span the cell membrane as a-helices. However, it is not known how often other structural elements occur in the transmembrane domains of receptors. Tlius, the presence of P-sheet structures, particularly in the case of receptors with complex structures, cannot be excluded (Hucho et al., 1994). 

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