Domain cytoplasmic

Fig. 1. The GP Ib-IX-V complex. The complex consists of seven transmembrane polypeptides denoted GP Iba (mol wt 145,000), GP IbP (mol wt 24,000), GPIX (mol wt 17,000) and GP V (mol wt 82,000), in a stoichiometry of 2 2 2 1. The hatched region represents the plasma membrane. The area above the hatched region represents the extracellular space that below represents the cytoplasm. The complex is a major attachment site between the plasma membrane and the cytoskeleton. Two molecules associated with the cytoplasmic domain are depicted a 14-3-3 dimer, which may mediate intracellular signaling, and actin-binding protein, which connects the complex to the cortical cytoskeleton and fixes its position and influences its function.

These interactions involve adhesion proteins called selectins, which are found both on the rolling leukocytes and on the endothelial cells of the vascular walls. Selectins have a characteristic domain structure, consisting of an N-terminal extracellular lectin domain, a single epidermal growth factor (EGR) domain, a series of two to nine short consensus repeat (SCR) domains, a single transmembrane segment, and a short cytoplasmic domain. Lectin domains, first characterized in plants, bind carbohydrates... 

FIGURE 10.10 The reaction of tridated sodium borohydride with the aspartyl phosphate at the active site of Na, K -ATPase. Acid hydrolysis of the enzyme following phosphorylation and sodium borohydride treatment yields a tripeptide containing serine, homoserine (derived from the aspartyl-phosphate), and lysine as shown. The site of phosphorylation is Asp" in the large cytoplasmic domain of the ATPase. 

Cadherins (Calcium-dependent adhesion proteins) are transmembrane proteins, which consist of an extracellular domain composed of cadherin-repeats, a transmembrane domain, and a cytoplasmic domain that interacts with catenins and/or other cytoplasmic proteins. 

Several nonconventional cadherins that contain cadherin repeats have been described but they have specific features not found in the classical cadherins [1]. The cadherin Flamingo, originally detected in Drosophila, contains seven transmembrane segments and in this respect resembles G protein-coupled receptors. The extracellular domain of Flamingo and its mammalian homologs is composed of cadherin repeats as well as EGF-like and laminin motifs. The seven transmembrane span cadherins have a role in homotypic cell interactions and in the establishment of cell polarity. The FAT-related cadherins are characterized by a large number of cadherin repeats (34 in FAT and 27 in dachsous). Their cytoplasmic domains can bind to catenins. T- (=truncated-)cadherin differs from other cadherins in that it has no transmembrane domain but is attached to the cell membrane via a glycosylpho-sphatidylinositol anchor. 

The cytoplasmic domains of protocadherins are unrelated to those of classical cadherins. They do not bind catenins and it is not clear whether they are associated with the cytoskeleton [1]. Some protocadherins interact with the c- src-related kinase Fyn, indicating a role in signal transduction (see below). 

Formation of vesicles is likely to require interaction with the soluble coat components of cytoplasmic domains of certain integral membrane proteins that may serve as... 

The vesicular acetylcholine transport can be inhibited by vesamicol and several related compounds. Vesami-col competitively inhibits transport by binding to a cytoplasmic domain on VAChT with a Kd of 5 nM. Vesamicol binding can be used to estimate transporter number, but neither vesamicol nor its analogues are currently used clinically. 

In addition to the membrane-inserted core domain of Kv channels, their cytoplasmic domains have important roles for Kv-channel function [5]. Many of these functions are related to subunits assembly, channel trafficking to and from the plasma membrane, and interactions with cytoskeletal components (Fig. la). A tetramerization (T) domain for subunit assembly has been well defined in Shaker-channels, where it is localized in the amino-terminus. Other Kv-channels (e.g., eag, HERG, KvLQTl) may have comparable domains within the cytoplasmic carboxy-terminus. ER retention and retrieval signals have been found... 

Thus, transition from Ei to E2 consists of an integrated structural change involving protection of bond C3 or T3 in the second cytoplasmic domain and exposure of T in the central domain, while the position of T2 in the N-terminus is altered relative to the central domain (Tj) so that cleavage of T2 becomes secondary to cleavage of T1 within the same a subunit in the E2 form. 

C3 cleavage is a selective and particularly efficient tool for examining structure-function relationships of the second cytoplasmic domain. Binding affinities for ADP and ATP are reduced 4-5-fold, while TNP-ATP binds with the same affinity as in native Na,K-ATPase. Nucleotide binding is not affected by or Rb although... 

Fig. 2. The hypothetical structure of Ca -ATPase. The structure consists of three major cytoplasmic domains, a pentahelical stalk region, and an intramembranous domain with ten, presumably helical, transmembrane segments. Ti and T2 mark the tryptic cleavage sites. Inset charged amino acids in and near the transmembrane region that may contribute to a Ca " -channel. Adapted from Brandi et al. [8].

The cytoplasmic domain of the Ca -ATPase of rabbit sarcoplasmic reticulum is very similar to the structure derived from Fourier-Bessel reconstructions of the Ca -ATPase tubules of scallop sarcoplasmic reticulum [176]. 

The cytoplasmic domains reconstructed from negatively stained [90] and from frozen-hydrated samples [91,177] have similar shapes. Both include the protruding lobe and the bridge region that links the Ca " -ATPase molecules into dimers. The intramembranous peptide domains of the two ATPase molecules which make up a dimer spread apart as they pass through the bilayer toward the luminal side of the membrane, establishing contacts with the Ca -ATPase molecules in the neighboring dimer chains. The lateral association of dimer chains into extended crystal lattice is... 

Fig. 4. Tentative allocation of probe binding sites within the three-dimensional structure of Ca -ATPase derived from vanadate-induced E2-type crystals. The top picture is the projection view of the Ca -ATPase down the x-axis, revealing the pear-shaped contours of ATPase molecules. The maximum length of the cytoplasmic domain to the tip of the lobe is =r65A. In the middle and bottom pictures the same structure is viewed down the x-axis, revealing the gap between the bridge and the bilayer surface and the connections between ATPase molecules in neighboring dimer chains. The proposed binding sites for lAEDANS and FITC are indicated. The bottom right picture is the same structure viewed down the y-axis. Adapted from Taylor et al. [90].

In the second view of the three-dimensional crystals the projected image normal to the plane of the lamellae (Fig. 7) shows ordered arrays of 40-50-A-diameter particles, that represent the cytoplasmic domains of ATPase molecules [156]. The crystals... 

The approximate location of the epitopes for more than 40 monoclonal anti-ATPase antibodies has been mapped to various regions within the cytoplasmic domain of the Ca " -ATPase [285,302-304]. All antibodies were found to bind with high affinity to denatured Ca -ATPase, but the binding to the native enzyme showed significant differences depending on the location of antigenic sites within the ATPase molecule. 

The slow and fast isoenzymes of Ca -ATPase contain 42 and 50 arginine residues, respectively. The C-terminal sequence of the neonatal fast-twitch isoenzyme is Arg-Arg-Lys. There are only four arginine residues in the putative transmembrane helices, which are probably located near the cytoplasmic or luminal surface of the membrane. The remaining arginine residues are distributed in the cytoplasmic domains. 

And third, since virtually all enzymes [67], particularly those that catalyze phos-phoryl-transfer reactions [68 74], possess structures with at least two, discrete, relatively rigid structural domains, or lobes, separated by a deep cleft, the cytoplasmic portion of the H -ATPase polypeptide chain in the model of Fig. 2 is drawn in such a way as to suggest this situation. The proposed interdomain cleft is indicated by the arrow. No additional structural features of the ATPase molecule are implied in the model. In regard to comparisons with the Ca -ATPase, it is of interest to note that the two cytoplasmic domains proposed in Fig. 2 correspond to the Cl and C2 domains in the model of Andersen and Vilsen [53]. 

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