Cell-associated receptors

Lanzrein M, Garred 0, Olsnes S, Sandvig K (1995) Diphtheria toxin endocytosis and membrane translocation is dependent on the intact membrane anchored receptor (HB-EGF precursor) Studies on the cell-associated receptor cleaved by a metalloprotease in TPA-treated cells. Biochem J 310 285-289. 

Figure 2. Mechanism of PDH. The three different subunits of the PDH complex in the mitochondrial matrix (E, pyruvate decarboxylase E2, dihydrolipoamide acyltrans-ferase Ej, dihydrolipoamide dehydrogenase) catalyze the oxidative decarboxylation of pyruvate to acetyl-CoA and CO2. E, decarboxylates pyruvate and transfers the acetyl-group to lipoamide. Lipoamide is linked to the group of a lysine residue to E2 to form a flexible chain which rotates between the active sites of E, E2, and E3. E2 then transfers the acetyl-group from lipoamide to CoASH leaving the lipoamide in the reduced form. This in turn is oxidized by E3, which is an NAD-dependent (low potential) flavoprotein, completing the catalytic cycle. PDH activity is controlled in two ways by product inhibition by NADH and acetyl-CoA formed from pyruvate (or by P-oxidation), and by inactivation by phosphorylation of Ej by a specific ATP-de-pendent protein kinase associated with the complex, or activation by dephosphorylation by a specific phosphoprotein phosphatase. The phosphatase is activated by increases in the concentration of Ca in the matrix. The combination of insulin with its cell surface receptor activates PDH by activating the phosphatase by an unknown mechanism.

CCR4 and CCR8 have been described as receptors preferentially expressed on Th2 cells (58,69,72,79,80). Just as for the Thl-associated chemokine receptors, despite the strong preferential association of CCR4 and CCR8 with Th2 cells, there is no evidence that these Th2-associated receptors are necessary for Th2 cell polarization (60,81). 

Qin S, Rottman JB, Myers P, et al. The chemokine receptors CXCR3 and CCR5 mark subsets of T cells associated with certain inflammatory reactions. J Clin Invest 1998 101 746-754. 

The synthesis and characterization of a somatostatin receptor-specific peptide H2N-(DPhe)-cyclo[Cys-Phe-(D-Trp)-Lys-Thr-Cys]-Thr-OH, labeled with an indo-dicarbo- and an indotricarbocyanine dye at the V-terminal amino group were described in [34], The ability of these fluorescent contrast agents to target the somatostatin receptor was demonstrated by flow cytometry in vitro, wherein the indotricarbocyanine conjugate led to elevated cell-associated fluorescence on somatostatin receptor-expressing tumor cells. The intracellular localization was visualized using NIR fluorescence microscopy. 

The other major class of extracellular LBPs of mammals is the lipocalins (Flower, 1996). These are approximately 20 kDa, P-sheet-rich proteins, performing functions such as the transport of retinol in plasma or milk, the capture of odorants in olfaction, invertebrate coloration, dispersal of pheromones, and solubilizing the lipids in tears (Flower, 1996). The retinol-binding protein (RBP) of human plasma is found in association with a larger protein, transthyretin, the complex being larger than the kidney threshold and thus not excreted, although the RBP itself may dissociate from the complex to interact with cell surface receptors in the delivery of retinol (Papiz et al., 1986 Sundaram et al., 1998). 

Nearly all of the interleukins are soluble molecules (one form of IL-1 is cell associated). They promote their biological response by binding to specific receptors on the surface of target cells. Most interleukins exhibit paracrine activity (i.e. the target cells are in the immediate vicinity of the producer cells), although some display autocrine activity (e.g. IL-2 can stimulate the growth and differentiation of the cells that produce it). Other interleukins display more systematic endocrine effects (e.g. some activities of IL-1). 

The signal transduction mechanisms by which most interleukins prompt their biological response are understood, in outline at least. In many cases, interleukin cell surface receptor binding is associated with intracellular tyrosine phosphorylation events. In other cases, serine and threonine residues of specific intracellular substrates are also phosphorylated. For some interleukins,... 

The unique domain following the SH4 domain from the N-terminus is a region with considerable variation in the amino acid sequences among different Src family members. This unique domain may be involved in protein-protein interactions. For example, the unique region of Lck is linked to CD4 and CD8, while those of Fyn and Lyn may be associated with the T- and B-cell antigen receptors. 

Estradiol activates a variety of signaling pathways via membrane-associated receptors in many cell types [20]. In neurons, rapid actions of estradiol stimulate translation of a key protein, PSD-95, involved in synapse formation [24],... 

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