Calcium shares the limelight in stimulus-secretion coupling

A ras-like protein is required for a post-Golgi event in yeast secretion

Secretion is blocked at the post-Golgi stage within 5 min of a shift of sec4-8 cells from 25 degrees C to 37 degrees C. Analysis of SEC4 predicts a protein product of 23.5 kd molecular weight that shares 32% homology with ras proteins and is essential for growth. The regions of best homology are those involved in the binding and hydrolysis of GTP. Duplication of SEC4 suppresses post-Golgi-blocked mutations in three sec genes. These mutations are lethal when combined with sec4-8 at 25 degrees C. Mutations that block elsewhere on the pathway are not suppressed by the SEC4 duplication and are not lethal when combined with sec4-8. We propose that the SEC4 product is a GTP-binding protein that plays an essential role in controlling a late stage of the secretory pathway.

Regulation of membrane turnover by ras proteins

Because ras oncogenes mediate abnormal cellular growth, ras proteins have been presumed to play a role primarily in growth control. The biological function of ras proteins may, however, prove to be much more diverse: ras proteins may be involved in cellular functions that control endocytosis and/or exocytosis.

The dual effector system for exocytosis in mast cells: obligatory requirement for both Ca2+ and GTP

The secretory process is a coordinated cellular response, initiated by occupation of surface receptors and comprising an ordered sequence of biochemical steps subject to multiple controls. Conceptually we can divide the sequence into two main sections comprising early, receptor-mediated events leading to generation of intracellular second messengers, and later events leading to membrane fusion and exocytosis. With the discovery that occupation of Ca2+ mobilising receptors leads to activation of polyphosphoinositide phosphodiesterase (PPI-pde) through the mediation of a G-protein (Gp), all the early events can be ascribed to the plasma membrane. Investigation of the exocytotic stage of secretion has been simplified by the use of permeabilised cells in which the composition of the cytosol can be precisely controlled. We have used streptolysin-O, a bacterial cytolysin which generates protein-sized pores in the plasma membrane, to investigate the exocytotic mechanism of rat mast cells. We find that in addition to the activation of PPI-dpe, GTP also acts in concert with Ca2+ at, or close to, the exocytotic site. Exocytosis can occur after substantial depletion of cytosol lactate dehydrogenase and 3-phosphoglycerate kinase indicating that soluble cytosol proteins are unlikely to play any role. There is no absolute requirement for ATP or phosphorylating nucleotide in exocytosis though when present the effective affinities of the two obligatory effectors (i.e. Ca2+ and GTP) are substantially enhanced.

Washout phenomena in dialyzed mast cells allow discrimination of different steps in stimulus-secretion coupling

Transient increases of intracellular calcium and exocytotic activity of rat peritoneal mast cells following stimulation with compound 48/80 were monitored using the Ca-indicator dye fura-2 and the capacitance measurement technique. It is known that mast cells very rapidly lose their secretory response towards antigenic or compound 48/80-induced stimulation in the whole-cell recording configuration of the patch-clamp technique due to "washout" of signal mediators. In contrast, we found that calcium transients remained unaffected by intracellular dialysis for as long as 10 min. The fast "washout" phenomenon of exocytosis could be overcome by supplementing the pipette filling solution with guanosinetriphosphate (GTP) indicating a major role for GTP-binding proteins in secretion. The restoration of exocytosis was transient and decayed within three minutes, suggesting diffusional escape of one or several other cytoplasmic substances involved in stimulus-secretion coupling. Quantitative aspects of this process and the implications of its differential effects on Ca-transients versus secretion are discussed.

Rat mast cells permeabilised with streptolysin O secrete histamine in response to Ca2+ at concentrations buffered in the micromolar range

Rat peritoneal mast cells have been permeabilised by treatment with streptolysin O which generates membrane lesions of macromolecular dimensions. In the presence of Ca2+ buffered at concentrations in the micromolar range, the permeabilised mast cells release histamine, beta-N-acetylglucosaminidase and lactate dehydrogenase. Release of the two secretory components (but not lactate dehydrogenase) has an obligatory requirement for a nucleoside triphosphate and micromolar concentrations of Ca2+. Inosine triphosphate (ITP) supports the release reaction better than ATP does. It is concluded that the secretory materials are released from the cells by an exocytotic mechanism, while lactate dehydrogenase leaks from the cells through the toxin-generated lesions. By initially withholding and then supplying Ca2+ to the permeabilised cells, it is shown that the exocytotic secretory reaction can persist even when the cytosol is depleted of the bulk of soluble proteins. The streptolysin O treated mast cell preparation represents a simplified system with which to study the mechanism of exocytosis.

Signal transduction by guanine nucleotide binding proteins

High affinity binding of guanine nucleotides and the ability to hydrolyze bound GTP to GDP are characteristics of an extended family of intracellular proteins. Subsets of this family include cytosolic initiation and elongation factors involved in protein synthesis, and cytoskeletal proteins such as tubulin (Hughes, S.M. (1983) FEBS Lett. 164, 1-8). A distinct subset of guanine nucleotide binding proteins is membrane-associated; members of this subset include the ras gene products (Ellis, R.W. et al. (1981) Nature 292, 506-511) and the heterotrimeric G-proteins (also termed N-proteins) (Gilman, A.G. (1984) Cell 36, 577-579). Substantial evidence indicates that G-proteins act as signal transducers by coupling receptors (R) to effectors (E). A similar function has been suggested but not proven for the ras gene products. Known G-proteins include Gs and Gi, the G-proteins associated with stimulation and inhibition, respectively, of adenylate cyclase; transducin (TD), the G-protein coupling rhodopsin to cGMP phosphodiesterase in rod photoreceptors (Bitensky, M.W. et al. (1981) Curr. Top. Membr. Transp. 15, 237-271; Stryer, L. (1986) Annu. Rev. Neurosci. 9, 87-119), and Go, a G-protein of unknown function that is highly abundant in brain (Sternweis, P.C. and Robishaw, J.D. (1984) J. Biol. Chem. 259, 13806-13813; Neer, E.J. et al. (1984) J. Biol. Chem. 259, 14222-14229). G-proteins also participate in other signal transduction pathways, notably that involving phosphoinositide breakdown. In this review, I highlight recent progress in our understanding of the structure, function, and diversity of G-proteins.

Calcium-binding proteins and secretion

The Ca ion plays a central role in the control of the regulated pathway of exocytotic secretion in eukaryote cells. Most secretagogues either directly or indirectly raise cytosolic free Ca levels which in turn affects granule biogenesis, contractile events, gel/sol transition in intracellular matrix and membrane fusion events occurring at exocytosis. Many of these responses are mediated by Ca-binding proteins among which calmodulin and protein kinase C have received prominent attention. Studies of the nature and inter-relationship of proteins which undergo Ca-dependent association with intracellular membranes in secretory tissue reveal that there may be further Ca-binding proteins in these cells which act as intracellular transducers of the Ca signal during secretion.

Suppression of triglyceride secretion by epinephrine in isolated rat hepatocytes

The effect of epinephrine on triglyceride synthesis and secretion was examined in isolated rat hepatocytes. Epinephrine potently inhibited triglyceride secretion but did not affect cellular triglyceride content or the rate of incorporation of radiolabelled glycerol into cell triglyceride. The inhibitory effect of epinephrine was abolished by inclusion of the alpha-adrenergic antagonist prazosin but not the beta-antagonist propranolol.