Transmembrane element Structure

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. 

Thus, the important question of the secondary structure of the transmembrane elements can only be addressed with models and by structural comparison with other transmembrane proteins for which the structure has been resolved. Detailed information on the structure of transmembrane elements is available for the photoreaction center of Rhodopseudomonas viridis (review Deisenhofer and Michel, 1989), cytochrome c oxidase (Iwata et al., 1995) and the OmpF porin of E. coli (Cowan et al., 1992 Fig. 5.3), amongst others. In addition, high resolution electron microscopic investigations and X-ray studies of bacteriorhodopsin, a light-driven ion pump with seven transmembrane elements, have yielded valuable information on the structure and configuration of membrane-spaiming elements (Henderson et al., 1990 Kimura et al., 1997 Pebay-Peyrula et al., 1997 Fig. 5.4). With the successful crystallization of the photoreaction center of Rhodopseudomonas viridis, a membrane protein was displayed at atomic resolution for the first time (Deisenhofer et al., 1985). The membrane-... 

Fig. 5.3. Structure of the OmpF porin of E. coli. The porin is a bacterial membrane protein with P-sheet structures as transmembrane elements. The structure of a monomer of the OmpF porin is shown. In total, 16 P-bands are configured in the form of a cylinder and form the waUs of a pore through which selective passage of ions takes place. LI—L8 are long loops, Tl,2,3 and T7,8 are short bends (T turn) that fink the P-sheets. According to Cowan et al. (1992), with per-...

Hie transmembrane domain may consist of one or several transmembrane elements (see also Fig. 5.2). In the latter case, these are arranged in the form of bundles, as shown in Figure 5.4 for bacteriorhodopsin. In the case of ion channels, in which several subunits are involved in formation of the transmembrane domain (see acetylcholine receptor, Fig.16.12), prediction of the structure of the membrane portion is very difficult. The different transmembrane elements are no longer equivalent in these cases. Part of the element is involved in formation of the irmer wall of the pore, other structural elements form the surface to the hydrophobic irmer of the phospholipid bilayer. It is evident that the polarity requirements for the amino acid side chains vary according to the position of the transmembrane elements (see Chapter 16). 

In these cases, how the different transmembrane elements associate to an ordered structured transmembrane domain can only be discussed based on electron microscopy and crosslinking experiments and affinity marking. Predictions based on these experiments are really more like models. 

The subunit structure of the cytokine receptors is very variable. Amongst the cytokine receptors, there are receptors composed of one polypeptide and receptors made up of two or three different polypeptide chains (Fig. 11.3), which therefore have a hetero-oligomeric structure. The receptors have one transmembrane element per polypeptide chain. The NH2 terminus is located in the extracellular region whilst the CO OH terminus is intracellular. 

Fig. 16.12. Subunit structure of the acetylcholine receptor, a) The acetylcholine receptor has the subunit structure 02 7 - The four transmembrane elements Ml—M4 are shown for the y subunit. The binding sites for acetylcholine (ACh) are located on the a-subunits. b) It is assumed that the inner wall of the ion pore is formed by M2 helices of the five subunits, c) Postulated configuration of the M2 helices in the narrowest region of the ion channel. In the closed state, five leucine residues (one per subunit) lie in the ion channel and hinder passage of ions. Above and below the block, there are negatively charged residues that serve as prefilters for ion passage.

High-resolution structural information about the transmembrane elements of transmembrane receptors could recently be obtained on the example of rhodopsin, the light-activated G protein coupled receptor of the vision process (Fig. 5.3). These data, together with earlier data on the structures of other transmembrane proteins (e.g., bacteriorhodopsin), have confirmed that a-helices are the principal structural building blocks of the transmembrane elements of membrane receptors. The transmembrane helices are composed of 20 - 30 hydrophobic amino acids with some polar... 

The key structural elements of the G protein-coupled receptors are seven sequence segments, each made up of 20-30 amino acids, that form a transmembrane domain, and span the membrane in the form of a-helices. The transmembrane elements are linked by loops of various sizes on the outer and inner side. The highest sequence homology of G protein-coupled receptors is found in the transmembrane elements, whilst the hydrophilic loop regions show stronger divergence between different receptors. 

Fig. 6.3 Model ofthe domain structure ofthe natriuretic peptide receptor NPR, a receptor-type guanylyl cyclase. NPR is a dimeric transmembrane receptor which spans the membrane with two transmembrane elements. The extracytosolic domain comprises the ligand binding site and contains several disulfide bridges. The cytosolic part is composed of a kinase homology domain with multiple phosphorylation sites, an ATP binding site of unknown function and the catalytic guanylyl cyclase domain.

Starting from the protein sequence (primary structure) several algorithms can be used to analyze the primary structure and to predict secondary structural elements like beta-strands, turns, and helices. The first algorithms from Chou and Fasman occurred already in 1978. The latest algorithms find e.g., that predictions of transmembrane... 

G-Protein coupled receptors (GPCR) represent the start element in secondary messenger producing systems. They comprise a family of over 1000 structurally-related members. These membrane proteins are also called serpentine or seven-helix receptors due to their seven transmembrane domains with an a-helical conformation. Receptors belonging to this class respond to a variety of hormones and neurotransmitters, and they detect odorant molecules or light [3,4]. 

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. 

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). 

Receptor tyrosine kinases are integral membrane proteins that have a hgand-binding domain on the extracellular side and a tyrosine kinase domain on the cytosohc side (see Fig. 8.1). The transmembrane portion is made up of just one structural element thus it is assumed that it crosses the membrane in an a-hehcal form. On the cytoplasmic side, in addition to the conserved tyrosine kinase domain, there are also further regulatory sequence portions at which autophosphorylation, and phosphorylation and dephosphorylation by other protein kinases and by protein phosphatases, can take place. 

It is generally agreed that activation of ion charmels is caused by a voltage-induced conformational change that opens a transmembrane pore. It is assumed that the ion charmel contains a structural element that can register changes in the electrical field. 

The system may be regarded as involving a Na+/Mg2+ co-catalysed phosphorylation step and a K+ catalysed dephosphorylation. Each phosphorylation/dephosphorylation step involves a pseudorotation of an Mg2+-stabilised 5-coordinate intermediate, resulting in transport of the alkali metal cations. The cation transport ability of the enzyme is a direct result of the enzymatic reactivity of the protein. There are three binding sites with high Na+ affinity and two with K+ affinity (occupied by Rb+ in the crystal structure determination). The structure (which is of the E2K state of the system) reveals that carboxy end of the a-subunit is held in a pocket in between transmembrane helices and acts as an unusual regulating element that controls sodium affinity and may be influenced by the membrane potential. 

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