Carboxy-terminal binding protein

Protein S. Protein S is a single-chain molecule of approximately 78,000 daltons that contains 10 y-carboxy glutamic acid residues in the NH -terminal portion of the molecule. Protein S is a regulatory vitamin K-dependent protein. In plasma 40% of this protein circulates free and 60% circulates bound to C4b binding protein. Free Protein S functions as a nonenzymatic cofactor that promotes the binding of Protein C to membrane surfaces (22—25). 

The crystal structures of DtxR and IdeR provide a detailed picture of this protein family (Figure 3.7, Plate 5). The N-terminal domain (residues 1-73) containing a helix-turn-helix motif binds a recognition nucleotide sequence of about 21 base pairs, as is nicely shown in a cocrystal of DNA and DtxR (Pohl et al., 1999). The central domain (74-140) has a function in dimerization the role of the third carboxy-terminal domain (141-230) is uncertain. Although metal-binding sites have been defined in these crystal structures, the mechanism by which metal binding causes the structural changes between apo- and holo-repressor is not clear. 

Rel proteins contain a 280-amino acid homology region, which mediates both DNA binding and dimerization. DNA contact regions are located in the amino-terminal half and the residues responsible for the dimerization reside in the carboxy terminal half. 

The protein folds into two domains, each of which possesses the Greek key /3-sheet folding topology (see Figs. 14 and 15). The amino-terminal domain binds a type I copper, while the carboxy-terminal domain does not bind a type I copper. A second metal site is found between domain 1 of one molecule in the trimer and domain 2 of a second molecule, so that, in all, six metals are bound by the trimer. The second metal is bound by two histidines from domain 1 and one histidine from domain 2. Density for a fourth ligand suggests a hydroxyl or water as that ligand. 

Detailed pictures of the iron-binding sites in transferrins have been provided by the crystal structures of lactoferrin (Anderson et ai, 1987, 1989 Baker etai, 1987) and serum transferrin (Bailey etal., 1988). Each structure is organized into two lobes of similar structure (the amino- and carboxy-terminal lobes) that exhibit internal sequence homology. Each lobe, in turn, is organized into two domains separated by a cleft (Fig. 3 and 10). The domains have similar folding patterns of the a//3 type. One iron site is present in each lobe, which occupies equivalent positions in the interdomain cleft. The same sets of residues serve as iron ligands to the two sites two tyrosines, one histidine, and one aspartate. Additional extra density completes the octahedral coordination of the iron and presumably corresponds to an anion and/or bound water. The iron sites are buried about 10 A below the protein surface and are inaccessible to solvent. 

Other long-range effects which may be exploited for metal binding are those involving the so-called macrodipoles of a helices, that is, the sizable electrostatic potentials that characterize the amino and carboxy termini of a helices. The amino-terminal portion of an a helix is characterized by a positive electrostatic potential and is implicated in the binding of phosphate anions to proteins (Hoi et al., 1978) the carboxy-terminal end of the helix is likewise characterized by a negative electrostatic potential and may be similarly implicated in cation binding. Work... 

Regarding the pH sensor, the carboxy tail length has been demonstrated as a determinant of pH sensitivity [Liu et al., 1993]. Further investigations [Morley et al., 1996] revealed a new model of intramolecular interactions in which the carboxy terminal serves as an independent domain that, under certain conditions, can bind to another separate domain of the connexin protein (e.g. a region including His-95) and close the channel, comparable to the ball-and-chain model for potassium channels. In this receptor (His-95),... 

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