The GTPase superfamily: conserved structure and molecular mechanism

GTPases are conserved molecular switches, built according to a common structural design. Rapidly accruing knowledge of individual GTPases--crystal structures, biochemical properties, or results of molecular genetic experiments--support and generate hypotheses relating structure to function in other members of the diverse family of GTPases.     

Regulation of antibody isotype secretion by subsets of antigen-specific helper T cells

The regulation of the subclass of immunoglobulin secreted by B cells has been studied in vitro in polyclonal systems using mitogens, such as lipopolysaccharide (LPS), to bypass the requirement for cognate interaction between antigen-specific T and B cells. In these systems, interleukin-(IL)-4 induces the secretion of IgG1 (ref. 1) and IgE (ref. 2); IL-5 enhances the secretion of IgA, and interferon-gamma (IFN-gamma) enhances the secretion of IgG2a (ref. 5). Clones of murine TH cells can be divided into two subsets, TH1 and TH2 (ref. 6). Both subsets synthesize IL-3 and granulocyte-monocyte colony-stimulating factor (GM-CSF), but only TH1 clones produce IL-2, IFN-gamma, and lymphotoxin (LT) and TH2 clones produce IL-4 and IL-5 (ref. 7). We have examined the role of clones of antigen-specific TH1 and TH2 cells in the regulation of the subclasses of IgG antibody secreted by antigen-specific B cells. Our results show that both types of TH cells induce the secretion of IgM and IgG3, whereas clones of TH1 and TH2 cells specifically induce antigen-specific B cells to secrete IgG2a and IgG1, respectively. We also demonstrate that regulation of commitment to the secretion of a particular IgG isotype occurs in two distinct stages: cognate interaction between T and B cells and interaction between T-cell-derived lymphokines and B cells.     

The apical surface of canine chief cell monolayers resists H+ back-diffusion

The resistance of the gastric mucosa to acid and peptic injury is reflected by a resistance to the back-diffusion of H+ from gastric lumen to blood. The nature of this 'barrier', however, remains undefined. Using Ussing chambers, we have now studied the acid-barrier function of monolayers prepared from dispersed canine fundic chief cells. These monolayers secrete pepsinogen in response to stimulation. We found that, on acidification of the apical solution to pH 2, transepithelial resistance (R) increased 2.6-fold and the monolayers maintained this 1:100,000 H+ concentration gradient for more than 4 h. The addition of aspirin to the acidified apical solution caused a rapid decay in R, as did acidification of the basolateral solution to a pH less than 5.5. Ouabain-treated monolayers displayed the rise in R expected with apical acidification, while potential difference (V) and short-circuit current (Isc) decreased essentially to zero, indicating impermeability to H+. However, if the integrity of the ouabain-treated monolayers was disrupted by low apical pH, H+ permeation occurred, reflected by an Isc that was dependent on the H+ gradient across monolayers. These data indicate that the apical surface of chief cells is a very tight barrier to H+ diffusion and may be an important element resisting acid-peptic injury.     

The GTPase superfamily: a conserved switch for diverse cell functions

Proteins that bind and hydrolyse GTP are being discovered at a rapidly increasing rate. Each of these many GTPases acts as a molecular switch whose 'on' and 'off' states are triggered by binding and hydrolysis of GTP. Conserved structure and mechanism in myriad versions of the switch--in bacteria, yeast, flies and vertebrates--suggest that all derive from a single primordial protein, repeatedly modified in the course of evolution to perform a dazzling variety of functions.     

Magnetic support of the optical emission line filaments in NGC 1275

The giant elliptical galaxy NGC 1275, at the centre of the Perseus cluster, is surrounded by a well-known giant nebulosity of emission-line filaments, which are plausibly in excess of 10(8) years old. The filaments are dragged out from the centre of the galaxy by radio-emitting 'bubbles' rising buoyantly in the hot intracluster gas, before later falling back. They act as markers of the feedback process by which energy is transferred from the central massive black hole to the surrounding gas. The mechanism by which the filaments are stabilized against tidal shear and dissipation into the surrounding extremely hot (4 x 10(7) K) gas has been unclear. Here we report observations that resolve thread-like structures in the filaments. Some threads extend over 6 kpc, yet are only 70 pc wide. We conclude that magnetic fields in the threads, in pressure balance with the surrounding gas, stabilize the filaments, so allowing a large mass of cold gas to accumulate and delay star formation.     

Structure of mammalian AMPK and its regulation by ADP

The heterotrimeric AMP-activated protein kinase (AMPK) has a key role in regulating cellular energy metabolism; in response to a fall in intracellular ATP levels it activates energy-producing pathways and inhibits energy-consuming processes. AMPK has been implicated in a number of diseases related to energy metabolism including type 2 diabetes, obesity and, most recently, cancer. AMPK is converted from an inactive form to a catalytically competent form by phosphorylation of the activation loop within the kinase domain: AMP binding to the γ-regulatory domain promotes phosphorylation by the upstream kinase, protects the enzyme against dephosphorylation, as well as causing allosteric activation. Here we show that ADP binding to just one of the two exchangeable AXP (AMP/ADP/ATP) binding sites on the regulatory domain protects the enzyme from dephosphorylation, although it does not lead to allosteric activation. Our studies show that active mammalian AMPK displays significantly tighter binding to ADP than to Mg-ATP, explaining how the enzyme is regulated under physiological conditions where the concentration of Mg-ATP is higher than that of ADP and much higher than that of AMP. We have determined the crystal structure of an active AMPK complex. The structure shows how the activation loop of the kinase domain is stabilized by the regulatory domain and how the kinase linker region interacts with the regulatory nucleotide-binding site that mediates protection against dephosphorylation. From our biochemical and structural data we develop a model for how the energy status of a cell regulates AMPK activity.     

Studies on the chemical composition of streptococcal Co-opsonin

Zusammenfassung Das natürliche an Bentonit adsorbierbare «Co-opsonin», welches für eine optimale Phagocytose virulenter Streptokokken der Gruppe A nötig ist, erweist sich als verschieden von Lysozym. Es liegen Hinweise vor, dass das «Co-opsonin» in der-Lipoprotein-Fraktion des Serums vorliegt.