Evidence for a model of exocytosis that involves calcium-activated channels.
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Calcium↗
Biomedical subjects
K E Krebs
Publications and source records attributed to K E Krebs.
The surface properties of apolipoproteins A-I and A-II at the lipid/water interface.
The monolayer system was employed to investigate the relative affinities of apolipoproteins A-I and A-II for the lipid/water interface. The adsorption of reductively 14C-methylated apolipoproteins to phospholipid monolayers spread at the air/water interface was determined by monitoring the surface pressure of the mixed monolayer and the surface concentration of the apoprotein. ApoA-II has a higher affinity than apoA-I for lipid monolayers; for a given initial surface pressure, apoA-II adsorbs more than apoA-I to monolayers of egg phosphatidylcholine (PC), distearoyl-PC and human high-density lipoprotein (HDL3) surface lipids. Comparison of the molecular packing of apolipoproteins A-I and A-II suggests that apoA-II adopts a more condensed conformation at the lipid/water interface compared to apoA-I. The ability of apoA-II to displace apoA-I from egg PC and HDL3 surface lipid monolayers was studied by following the adsorption and desorption of the reductively 14C-methylated apolipoproteins. At saturating subphase concentrations of the apoproteins (3.10(-5) g/100 ml), two molecules of apoA-II absorbed for each molecule of apoA-I displaced. This displacement was accompanied by an increase in surface pressure. An identical stoichiometry for the displacement of apoA-I from HDL particles by apoA-II has been reported by others. At low subphase concentrations of apoproteins (5.10(-6) g/100 ml), the apoA-I/lipid monolayer was not fully compressed and could accommodate the adsorbing apoA-II molecules without displacement of apoA-I molecules. ApoA-I molecules were unable to displace apoA-II from the lipid/water interface. The average residue hydrophobicity of apoA-II is higher than that of apoA-I; this may contribute to the higher affinity of apoA-II for lipids compared to apoA-I. The probable helical regions in apolipoproteins A-I and A-II were located using a secondary structure prediction algorithm. The analysis suggests that the amphiphilic properties of the alpha-helical regions of apoA-I and apoA-II are probably not significantly different. Further understanding of the differences in surface activity of these apolipoproteins will require more knowledge of their secondary and tertiary structures.
Amino Acid Sequence↗
A comparison of the surface activities of human apolipoproteins A-I and A-II at the air/water interface.
Surface pressure (pi) and adsorption isotherms for human apolipoproteins A-I and A-II at the air/water interface have been determined and used to deduce the probable molecular structures of the monomolecular films. The surface concentrations were measured using the surface radioactivity method to monitor the adsorption of reductively [14C]methylated apoproteins. Apolipoprotein A-I and apolipoprotein A-II are extremely surface-active proteins and adsorb to exert maximal pi values of 22 and 24 mN.m-1 respectively, at a steady-state subphase concentration of about 3.10(-5) g/100 ml (equivalent to 11 and 17 nM for apolipoprotein A-I and apolipoprotein A-II, respectively). At saturation monolayer coverage, the average molecular areas for apolipoprotein A-I and apolipoprotein A-II are 15 and 13 A2/residue, respectively. These packing densities are consistent with monolayers consisting largely of alpha-helical protein molecules lying with the long axes of the helical segments in the plane of the interface. Comparison of the molecular packings of spread and adsorbed monolayers of these proteins indicates that at low pi values, the adsorbed films are more expanded, but at high pi values, the molecular packing in both types of film is the same.
Adsorption↗
Brain spectrin(240/235) and brain spectrin(240/235E): conservation of structure and location within mammalian neural tissue.
We demonstrate that the brain spectrin isoforms (240/235) and (240/235E) are present in all mammalian species studied (human, bovine, mouse, and rat). Immunohistochemistry with a panel of eleven polyclonal antibodies have indicated an identical localization of the brain spectrin isoforms in all mammalian species. Brain spectrin(240/235) is found primarily in axons, and brain spectrin(240/235E) primarily in cell bodies and dendrites. Immunoprecipitation and Western blotting studies have indicated that the subunit molecular weights of brain spectrin(240/235) and (240/235E) are identical in all mammalian species. We demonstrate that when proteolysis is not completely blocked during immunoprecipitation studies, the 235 kDa subunits are converted to a 230 kDa polypeptide [brain spectrin(240/235)] and a 232 kDa polypeptide [brain spectrin(240/235E)]. Finally, we show that both the alpha and beta subunits of brain spectrin(240/235) and brain spectrin(240/235E) are antigenically distinct in every species examined. These studies indicate that previous findings on the structure, location, and function of mouse brain spectrin isoforms can now be generalized to all mammalian species.
Aged↗
Spectrin and related molecules.
This review begins with a complete discussion of the erythrocyte spectrin membrane skeleton. Particular attention is given to our current knowledge of the structure of the RBC spectrin molecule, its synthesis, assembly, and turnover, and its interactions with spectrin-binding proteins (ankyrin, protein 4.1, and actin). We then give a historical account of the discovery of nonerythroid spectrin. Since the chicken intestinal form of spectrin (TW260/240) and the brain form of spectrin (fodrin) are the best characterized of the nonerythroid spectrins, we compare these molecules to RBC spectrin. Studies establishing the existence of two brain spectrin isoforms are discussed, including a description of the location of these spectrin isoforms at the light- and electron-microscope level of resolution; a comparison of their structure and interactions with spectrin-binding proteins (ankyrin, actin, synapsin I, amelin, and calmodulin); a description of their expression during brain development; and hypotheses concerning their potential roles in axonal transport and synaptic transmission.
Animals↗
Brain protein 4.1 subtypes: a working hypothesis.
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Animals↗
The 180-kD component of the neural cell adhesion molecule N-CAM is involved in cell-cell contacts and cytoskeleton-membrane interactions.
N-CAM180, the molecular form of the three neural cell adhesion molecules (N-CAM) with the largest cytoplasmic domain, is accumulated at sites of cell-cell contact (cell bodies, neurites, growth cones) in cultures of neuroblastoma and cerebellum. At these sites the cytoskeleton-membrane linker protein brain spectrin and actin are also accumulated. Brain spectrin copurifies with N-CAM180 by immunoaffinity chromatography and binds specifically to N-CAM180 but not to N-CAM140 or N-CAM120 in a solid-phase binding test. These observations indicate an association of N-CAM180 with the cytoskeleton in vivo. This association may underlie the reduced lateral mobility of N-CAM180 in the surface membrane compared to N-CAM140 (Pollerberg et al. 1986). Together with the fact that N-CAM180 is only expressed after termination of neuron migration in vivo (Persohn and Schachner, unpublished) these results suggest a role for N-CAM180 in stabilization of cell contacts.
Animals↗
Amelin and synapsin I are 4.1 related spectrin binding proteins in brain.
How do synaptic vesicles move towards the presynaptic plasma membrane, fuse with that membrane, and release their contents during synaptic transmission? The answers to these questions at the molecular level are just beginning to be understood. Synapsin I is a neuron specific phosphoprotein that is associated with the cytoplasmic surface of synaptic vesicles. During synaptic transmission, the translocation of the synaptic vesicles to the presynaptic membrane of the neuron is thought to be mediated through changes in the phosphorylation state of synapsin I. It has been suggested that synapsin I is a spectrin binding protein related to the erythrocyte cytoskeletal protein 4.1, which binds to the terminal ends of the erythrocyte spectrin tetramer. The interaction of synapsin I (through brain spectrin) with the neuronal cytoskeleton may be essential for regulating the movement of synaptic vesicles towards the presynaptic plasma membrane. In addition, we have identified another protein in brain that is immunologically and structurally more closely related to erythrocyte 4.1 than is synapsin I. This protein, termed amelin, is localized in the cell body and dendrites of the neuron, whereas synapsin I is found exclusively in the synaptic terminals, suggesting that there is a family of erythrocyte 4.1 related proteins present in brain with distinct subcellular distribution and functions.
Animals↗
Structural and functional relationship of red blood cell protein 4.1 to synapsin I.
It has been suggested that the neuron specific protein synapsin I is closely related to red blood cell (rbc) protein 4.1. A systematic comparison of the structural and functional properties of rbc protein 4.1 and synapsin I has been carried out. There is approximately a three order of magnitude difference in cross reactivity of synapsin I with rbc 4.1 antiserum vs. synapsin I antiserum, as determined by a competitive quantitative dot assay. Two-dimensional chymotryptic iodopeptide mapping analysis demonstrated limited peptide homology (approximately 34% spot overlap) between rbc 4.1 and synapsin I. Dephosphorylated synapsin I binds saturably to brain spectrin (240/235) with an estimated dissociation constant (Kd) of 700 nM and a maximal binding capacity of 4 mol synapsin I/mol spectrin tetramer, similar to the affinity and stoichiometry of 4.1 binding to rbc spectrin. Synapsin I was found to bind to the terminal ends of the brain spectrin tetramer by low-angle rotary shadowing, analogous to 4.1 binding to rbc spectrin. In summary, synapsin I is structurally and immunologically distinct from rbc 4.1, yet shares functional similarities with rbc 4.1 with respect to its spectrin binding characteristics.
Animals↗
Amelin: a 4.1-related spectrin-binding protein found in neuronal cell bodies and dendrites.
An immunoreactive, structural, and functional analog of erythrocyte protein 4.1 is present in neuronal cell bodies and dendrites. Other investigators have described the isolation of a 4.1 analog in brain with structural characteristics suggesting that its identity was synapsin I, a neuronal phosphoprotein localized in the presynaptic terminal in association with small synaptic vesicles. In this report we demonstrate that the cell body/dendritic form of brain protein 4.1, which we have named amelin, is distinct from that of synapsin I on the basis of subcellular localization, migration in 2-dimensional gel electrophoresis, and structural criteria. We also demonstrate that amelin, like synapsin I, can bind brain spectrin on nitrocellulose paper. Neither amelin nor synapsin I binds calmodulin, as determined by a blot binding assay. We hypothesize that there exists in brain a family of 4.1-related proteins with distinct subcellular localization and function.
Animals↗
Studies of apolipoproteins at the air-water interface.
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Adsorption↗
A rapid purification of synapsin I: a neuron specific spectrin binding protein.
We have developed a one chromatographic step isolation protocol for the neuron specific protein synapsin I. This procedure results in a yield of 80 micrograms/g brain, which is ten fold better than the highest yield yet reported for this protein. The authenticity of the synapsin I isolated by this procedure is demonstrated by comigration with authentic synapsin I on SDS-polyacrylamide gels, crossreactivity with antibody specific against synapsin I, and nearly identical two dimensional chrymotryptic iodopeptide maps of authentic synapsin I and the protein purified by this protocol. Synapsin I isolated by this procedure retains its functional properties, demonstrated by the ability of synapsin I to stimulate the formation of a brain spectrin(240/235)/synapsin I/F-actin ternary complex as determined by a low shear falling ball viscometry assay. This novel protocol therefore has the advantage of being a rapid, high yield procedure that retains the functional properties of synapsin I.
Actins↗
Occurrence of spectrin-like protein in Y-1 adrenal tumor cells.
With the aid of two monospecific antibodies raised in rabbits (antimouse erythrocyte spectrin and antimouse brain spectrin), the presence of a spectrin-like protein was demonstrated in mouse adrenal tumor (Y-1) cells. Y-1 cells contain two large polypeptides, with mol wt characteristic of nonerythroid spectrin alpha- and beta-subunits (240,000 and 235,000). When proteins from plasma membranes of Y-1 cells were electrophoretically transferred to a nitrocellulose membrane, two polypeptides with mol wt of 240,000 and 225,000 were specifically stained with antimouse erythrocyte (rbc) spectrin immunoglobulin G (IgG). The rbc spectrin antibody was used to immunoprecipitate Y-1 spectrin from a neutral detergent (physiological ionic strength) cell extract. The 240,000 (alpha)- and 235,000 (beta)-dalton polypeptides were immunoprecipitated in a 1:1 molar ratio, despite the fact that the antibody recognizes only the alpha-subunit. Two-dimensional chymotryptic peptide-mapping analysis indicated that the 240,000- and 235,000-dalton subunits of Y-1 adrenal tumor spectrin are structurally unique and share limited homology with mouse rbc spectrin alpha- and beta-subunits, but are nearly identical to the mouse brain spectrin 240,000-dalton alpha-subunit and 235,000-dalton beta-subunit. Indirect immunofluorescence with anti-rbc or antibrain spectrin IgG and goat antirabbit IgG conjugated with rhodamine demonstrated intense staining at the plasma membrane and throughout the cytoplasm of Y-1 cells, with little staining within the nucleus.
Adrenal Gland Neoplasms↗
The contribution of alpha-helices to the surface activities of proteins.
The amphilicity of an alpha-helical segment in a protein may be quantitated by calculating its mean helical hydrophobic moment (mu H). For proteins whose hydrophobic interactions with interfaces are mediated by alpha-helices, the surface pressures exerted at the air-water interface correlate with the product (mu H X F) where mu H is the mean helical hydrophobic moment averaged over all helices in the entire molecule, and F is the fraction of alpha-helix in the protein. Knowledge of mu H permits a description of the contribution of amphipathic alpha-helices to the surface activities at the air-water interface of serum apolipoproteins, surface-seeking peptides, and globular water-soluble proteins.
Animals↗
The helical hydrophobic moments and surface activities of serum apolipoproteins.
The mean helical hydrophobic moments (muH) have been used to compare the amphipathic helices of several apolipoprotein classes with the helices in membrane proteins, water-soluble globular proteins and surface-active peptides. The amphipathic helices in serum apolipoproteins have similar muH and mean hydrophobicities to helices in water-soluble globular proteins. The intrinsic surface activities of proteins and peptides, as determined by surface pressure at the air/water interface, correlate with the product (muH . F) where muH is the average value of muH for all helices in the molecule, and F is the fraction of alpha-helix structure in the protein.
Apolipoproteins↗
A comparison of the surface activities of rat plasma apolipoproteins C-II, C-III-0, C-III-3.
Rat apolipoproteins C-II, C-III-0 and C-III-3 give similar surface pressure (pi)-molecular area isotherms when spread at the air/water interface. When allowed to adsorb to the clean air/water interface, the intrinsic surface activity of apolipoprotein C-II is somewhat higher than that of apolipoprotein C-III. All three apolipoprotein C molecules can penetrate an egg phosphatidylcholine monolayer spread at the air/water interface causing an increase (delta pi) in surface pressure. Increasing the initial surface pressure (pi i) of the lipid monolayer decreases delta pi, and delta pi = 0 when pi i greater than or equal to 32 +/- 2 mN . m-1 for all three apolipoprotein C proteins. This implies that apolipoproteins C-II, C-III-0 and C-III-3 would adsorb and desorb similarly from the surfaces of lipoprotein particles during metabolism.
Adsorption↗