Aspartate receptors of Escherichia coli and Salmonella typhimurium bind ligand with negative and half-of-the-sites cooperativity

Heparin-induced oligomerization of FGF molecules is responsible for FGF receptor dimerization, activation, and cell proliferation

Heparin is required for fibroblast growth factor (FGF) stimulation of biological responses. Using isothermal titration calorimetry, we show that acidic FGF (aFGF) forms a 1:1 complex with the soluble extracellular domain of FGF receptor (FGFR). Heparin exerts its effect by binding to many molecules of aFGF. The resulting aFGF-heparin complex can bind to several receptor molecules, leading to FGFR dimerization. In two cell lines lacking endogenous heparan sulfate, exogenous heparin is required for FGFR dimerization, tyrosine kinase activation, c-fos mRNA transcription, and cell proliferation. Moreover, a synthetic heparin analog that binds monovalently to aFGF blocks FGFR dimerization, activation, and signaling via FGFR. We propose that heparin causes oligomerization of aFGF such that its binding to FGFR results in dimerization and activation. This represents a novel mechanism for transmembrane signaling and may account for the action of many heparin-bound growth factors.

Transmembrane signalling and the aspartate receptor

BACKGROUND

The aspartate receptor is a transmembrane protein that mediates bacterial chemotaxis. The structures of the periplasmic ligand-binding domain reveal a dimer, each subunit with four alpha-helix bundles, with aspartate binding to one of two sites at the subunit interface. The transmembrane regions of the receptor were not included in these structures.

RESULTS

To investigate the structure of the transmembrane region, we have made a mutant protein with two cross-links, restraining the subunit-subunit interface on both sides of the membrane, and have made an energy-minimized model of the transmembrane region. We demonstrate that the transmembrane helices form a coiled coil which extends from the periplasmic subunit through the membrane. We have constructed a model of the ligand-binding domains with the amino-terminal transmembrane helices.

CONCLUSIONS

We draw three conclusions from our model. Firstly, the interface between receptor subunits in the intact receptor consists of an uninterrupted coiled coil. Secondly, this structure rules out several postulated mechanisms of signalling. Thirdly, side chain packing constraints within the helices dictate that local structural changes must be small, but are propagated over a long distance rather than being dissipated locally. Low energy changes in the conformation of side chains are a probable mechanism of signal transduction in the aspartate receptor.

Chemosensing and signal transduction in bacteria

Major advances have been made over the past year in understanding the molecular mechanisms involved in membrane receptor function, and in resolving the global organization of intracellular signaling pathways. Crystallographic and biochemical studies are revealing details of transmembrane signaling mechanisms and the phosphorylation reactions of the two-component regulatory systems. In addition, the discovery of new signal transduction pathways and new inputs into known pathways are providing a clearer view of the basic architecture of the signal transduction networks within the bacterial cell.

Attractant- and disulfide-induced conformational changes in the ligand binding domain of the chemotaxis aspartate receptor: a 19F NMR study

The isolated ligand binding domain of the chemotaxis aspartate receptor is the focus of the present study, which both (a) identifies structural regions involved in the attractant-induced conformational change and (b) investigates the kinetic parameters of attractant binding. To analyze the attractant-induced conformational change within the homodimeric domain, 19F NMR is used to monitor six para-fluorophenylalanine (4-F-Phe) positions within each identical subunit of the homodimer. The binding one molecule of aspartate to the homodimer perturbs three of the 4-F-Phe resonances significantly: 4-F-Phe150 in the attractant binding site, 4-F-Phe107 located 26 A from the site, and 4-F-Phe180 at a distance of 40 A from the site. Comparison of the frequency shifts triggered by aspartate and glutamate reveals that these attractants generate different conformations in the vicinity of the attractant site but trigger indistinguishable long-range conformational effects at distant positions. This long-range conformational change is specific for attractant binding, since formation of the Cys36-Cys36' disulfide bond or the nonphysiological binding of 1,10-phenanthroline to an aromatic pocket distal to the attractant site each yield conformational changes which are significantly more localized. The attractant-triggered perturbations detected at 4-F-Phe107 and 4-F-Phe180 indicate that the structural change includes an intrasubunit component communicated through the domain to its C-terminal region, which, in the full-length receptor, continues through the membrane as the second membrane-spanning helix. It would thus appear that the transmembrane signal is transmitted through this helix.(ABSTRACT TRUNCATED AT 250 WORDS)