Membrane spanning proteins

Cell membrane spanning proteins contain a luminal/ extracellular domain, a transmembrane region and a cytosolic domain. In a type I transmembrane protein the N-terminus is the extracellular/luminal part of the protein, whereas the C-terminus comprises the cytosolic region of the membrane protein. 

Calcium channels are members of the large family of proteins, including Na and channels, which become incorporated into plasma membranes, and which form intermittent aqueous pathways through which ions can move. The channels open and close. As is the case generally for membrane spanning proteins, a Ca channel is formed by a set of helical units, in this case seven, which associate to form the channel. 

The movement of solutes from the external environment into the cell is usually achieved using cell membrane-spanning proteins that facilitate solute transfer. These are necessary, since most solutes (e.g. sugars, amino acids, salts) will not readily diffuse through the hydrophobic cell membrane. Movement of solutes into the epithelial cell can involve a variety of protein carriers or channels including (see Figure 1) ... 

Symporters are membrane-spanning proteins which translocate two or more solutes in the same direction across the cell membrane. In epithelial tissue, such... 

Na+K+-ATPase a ubiquitous membrane-spanning protein, which uses ATP as a source of energy for moving ions, responsible for Na+ flux against the electrochemical gradient. 

Owing to the small number of known peroxisomal membrane proteins, the signals (mPTSs) that direct them to the membrane are not well characterized. In one example, mPTS is found in the fourth loop (20 residues) of a 6 membrane-spanning protein. The loop faces to the matrix side. In another example, mPTS was found in the N-terminal 40-residue segment on the matrix side. Both of them contain a 5-residue stretch rich in basic amino acids and seem to be a part of the mPTS. 

Hartmann, E., Rapoport, T., and Lodish H. (1989). Predicting the orientation of eukaryotic membrane-spanning proteins. Proc. Natl. Acad. Sci. U.S.A. 86, 5786-5790. 

Klein, P., Kanehisa, M., and DeLisi, G. (1985). The detection and classification of membrane-spanning proteins. Biochim. Biophys. Acta 815, 468—476. 

The superfamily of seven-transmembrane C-protein-coupled receptors is the largest and most diverse group of membrane-spanning proteins [172]. Within all identified human genes, approximately 1,000 encode C-protein-coupled receptors. Many... 

Many proteins will not yield crystals in initial crystal trials. Unfortunately, crystallization is still a trial-and-error procedure, with no real way of predicting success or failure. If you fail to get crystals in your initial trials, it need not be the end of your structural studies (although it could be ). Many of the targets for neuroscientists are going to be membrane-bound or membrane-spanning proteins, and these are notoriously difficult to crystallize. Techniques are continuously being developed and refined to improve our ability to crystallize difficult protein examples. 

All these pathways converge to modulate the activity of the enzyme, H+/K+ ATPase, the proton pump of the parietai ceii. This is a membrane-spanning protein, an ATP-dependent ion pump, exchanging K+ and H+ ions. 

While one end of the dystrophin molecule binds to actin filaments, the C-terminal domain associates with several additional proteins to form a dystrophin-glycoprotein complex (see figure)/1 k Dystrophin is linked directly to the membrane-spanning protein P-dystroglycan, which in the outer membrane surfaces associates with a glycoprotein a-dystroglycan. The latter binds to laminin-2 (Fig. 8-33), a protein that binds the cell to the basal lamina. Four... 

All of the growth factors characterized so far are proteins (table 24.7). Many of them are cleaved from larger precursors. The 53-amino-acid epidermal growth factor (EGF) is cleaved from a precursor of 1,168 amino acids. This precursor is a membrane-spanning protein, with the EGF moiety and nine related sequences in the extracellular domain. EGF is homologous to several other growth factors,... 

MHC class II molecules are made up of two membrane-spanning proteins each chain has a size of 30 kDa and is made up of two globular domains. These domains are called a-1, a-2, (3-1 and P-2. Each chain possesses an immunoglobulin-like region next to the cell membrane. MHC class II molecules are highly polymorphic. 

Synaptotagmins comprise a small family of single-membrane spanning proteins that are expressed in neurons and neuroendocrine cells. So far 16 members have been identified in vertebrates (Craxton 2004). They contain an N-terminal transmembrane domain followed by a variable linker region and two C2 domains which are connected by a short linker (Stidhof 2002). Some synaptotagmins have additional short... 

The dominance of changes at the sn-2 position in PC may reflect the importance of viscotropic relationships. It has been proposed that a high degree of unsaturation at this position could contribute to the abilities of membrane-spanning proteins to undergo conformational changes (Litman and Mitchell, 1996). 

The mathematical equations describing the ionic currents are empirical, but there is now proof for the existence of membrane-spanning proteins that act as channels for sodium and potassium ions and that open and close in response to changes in membrane polarization. 

---