Solution structure of the antiapoptotic protein bcl-2

The structures of two isoforms of Bcl-2 that differ by two amino acids have been determined by NMR spectroscopy. Because wild-type Bcl-2 behaved poorly in solution, the structures were determined by using Bcl-2/Bcl-x(L) chimeras in which part of the putative unstructured loop of Bcl-2 was replaced with a shortened loop from Bcl-x(L). These chimeric proteins have a low pI compared with the wild-type protein and are soluble. The structures of the two Bcl-2 isoforms consist of 6 alpha-helices with a hydrophobic groove on the surface similar to that observed for the homologous protein, Bcl-x(L). Comparison of the Bcl-2 structures to that of Bcl-x(L) shows that although the overall fold is the same, there are differences in the structural topology and electrostatic potential of the binding groove. Although the structures of the two isoforms of Bcl-2 are virtually identical, differences were observed in the ability of the proteins to bind to a 25-residue peptide from the proapoptotic Bad protein and a 16-residue peptide from the proapoptotic Bak protein. These results suggest that there are subtle differences in the hydrophobic binding groove in Bcl-2 that may translate into differences in antiapoptotic activity for the two isoforms.

Rationale for Bcl-xL/Bad peptide complex formation from structure, mutagenesis, and biophysical studies

The three-dimensional structure of the anti-apoptotic protein Bcl-xL complexed to a 25-residue peptide from the death promoting region of Bad was determined using NMR spectroscopy. Although the overall structure is similar to Bcl-xL bound to a 16-residue peptide from the Bak protein (Sattler et al., 1997), the Bad peptide forms additional interactions with Bcl-xL. However, based upon site-directed mutagenesis experiments, these additional contacts do not account for the increased affinity of the Bad 25-mer for Bcl-xL compared to the Bad 16-mer. Rather, the increased helix propensity of the Bad 25-mer is primarily responsible for its greater affinity for Bcl-xL. Based on this observation, a pair of 16-residue peptides were designed and synthesized that were predicted to have a high helix propensity while maintaining the interactions important for complexation with Bcl-xL. Both peptides showed an increase in helix propensity compared to the wild-type and exhibited an enhanced affinity for Bcl-xL.

Crystal structure and mutagenic analysis of the inhibitor-of-apoptosis protein survivin

The coupling of apoptosis (programmed cell death) to the cell division cycle is essential for homeostasis and genomic integrity. Here, we report the crystal structure of survivin, an inhibitor of apoptosis, which has been implicated in both control of cell death and regulation of cell division. In addition to a conserved N-terminal Zn finger baculovirus IAP repeat, survivin forms a dimer through a symmetric interaction with an intermolecularly bound Zn atom located along the molecular dyad axis. The interaction of the dimer-related C-terminal alpha helices forms an extended surface of approximately 70 A in length. Mutagenesis analysis revealed that survivin dimerization and an extended negatively charged surface surrounding Asp-71 are required to counteract apoptosis and preserve ploidy. These findings may provide a structural basis for a dual role of survivin in inhibition of apoptosis and regulation of cell division.

Crystal structure and mutagenic analysis of the inhibitor-of-apoptosis protein survivin

The coupling of apoptosis (programmed cell death) to the cell division cycle is essential for homeostasis and genomic integrity. Here, we report the crystal structure of survivin, an inhibitor of apoptosis, which has been implicated in both control of cell death and regulation of cell division. In addition to a conserved N-terminal Zn finger baculovirus IAP repeat, survivin forms a dimer through a symmetric interaction with an intermolecularly bound Zn atom located along the molecular dyad axis. The interaction of the dimer-related C-terminal alpha helices forms an extended surface of approximately 70 A in length. Mutagenesis analysis revealed that survivin dimerization and an extended negatively charged surface surrounding Asp-71 are required to counteract apoptosis and preserve ploidy. These findings may provide a structural basis for a dual role of survivin in inhibition of apoptosis and regulation of cell division.

Crystal structure and mutagenic analysis of the inhibitor-of-apoptosis protein survivin

The coupling of apoptosis (programmed cell death) to the cell division cycle is essential for homeostasis and genomic integrity. Here, we report the crystal structure of survivin, an inhibitor of apoptosis, which has been implicated in both control of cell death and regulation of cell division. In addition to a conserved N-terminal Zn finger baculovirus IAP repeat, survivin forms a dimer through a symmetric interaction with an intermolecularly bound Zn atom located along the molecular dyad axis. The interaction of the dimer-related C-terminal alpha helices forms an extended surface of approximately 70 A in length. Mutagenesis analysis revealed that survivin dimerization and an extended negatively charged surface surrounding Asp-71 are required to counteract apoptosis and preserve ploidy. These findings may provide a structural basis for a dual role of survivin in inhibition of apoptosis and regulation of cell division.

Amyloid-beta aggregation: selective inhibition of aggregation in mixtures of amyloid with different chain lengths

One of the clinical manifestations of Alzheimer's disease is the deposition of the 39-43 residue amyloid-beta (A beta) peptide in aggregated fibrils in senile plaques. Characterization of the aggregation behavior of A beta is one of the critical issues in understanding the role of A beta in the disease process. Using solution hydrodynamics, A beta was observed to form three types of species in phosphate-buffered saline: insoluble aggregates with sedimentation coefficients of approximately 50,000 S and molecular masses of approximately 10(9) Da, "soluble aggregates" with sedimentation coefficients of approximately 30 S and masses of approximately 10(6) Da, and monomer. When starting from monomer, the aggregation kinetics of A beta 1-40 (A beta 40) and A beta 1-42 (A beta 42), alone and in combination, reveal large differences in the tendency of these peptides to aggregate as a function of pH and other solution conditions. At pH 4.1 and 7.0-7.4, aggregation is significantly slower than at pH 5 and 6. Under all conditions, aggregation of the longer A beta 42 was more rapid than A beta 40. Oxidation of Met-35 to the sulfoxide in A beta 40 enhances the aggregation rate over that of the nonoxidized peptide. Aggregation was found to be dependent upon temperature and to be strongly dependent on peptide concentration and ionic strength, indicating that aggregation is driven by a hydrophobic effect. When A beta 40 and A beta 42 are mixed together, A beta 40 retards the aggregation of A beta 42 in a concentration-dependent manner. Shorter fragments have a decreasing ability to interfere with A beta 42 aggregation. Conversely, the rate of aggregation of A beta 40 can be significantly enhanced by seeding slow aggregating solutions with preformed aggregates of A beta 42. Taken together, the inhibition of A beta 42 aggregation by A beta 40, the seeding of A beta 40 aggregation by A beta 42 aggregates, and the chemical oxidation of A beta 40 suggest that the relative abundance and rates of production of different-length A beta and its exposure to radical damage may be factors in the accumulation of A beta in plaques in vivo.

Rotational diffusion of band 3 in erythrocyte membranes. 2. Binding of cytoplasmic enzymes

Time-resolved phosphorescence anisotropy has been used to study the rotational diffusion of eosin-labeled human erythrocyte band 3 in the presence of an enzyme bound at its cytoplasmic pole. With increasing amounts of G3PD (glyceraldehyde-3-phosphate dehydrogenase) added to ghosts, the infinite time anisotropy (r infinity) increases, and at saturating concentrations, very little decay of the anisotropy r(t) occurs at all. These phenomena are reversed by elution of the enzyme with 150 mM NaCl. Prior proteolytic removal of the N-terminal 41-kDa cytoplasmic fragment of band 3 also disenables the G3PD effect. When ghosts are stripped of their residually bound G3PD, a small reduction in the fraction of immobile band 3 is observed. Finally, titration of band 3 sites with aldolase shows effects on the r(t) qualitatively similar to those observed with G3PD. On the basis of our interpretation of the heterogenous anisotropy decay of eosin-labeled band 3 [Matayoshi, E. D., & Jovin, T. M. (1991) Biochemistry (preceding paper in this issue)], we conclude that the binding of G3PD and aldolase results in the immobilization of band 3 oligomers.

Rotational diffusion of band 3 in erythrocyte membranes. 1. Comparison of ghosts and intact cells

The rotational diffusion of eosin-labeled 3 in human erythrocyte cells and hemoglobin-free ghosts at 37 degrees C has been studied in detail by polarized delayed luminescence. The time-resolved anisotropy with both cells and freshly prepared ghosts is similar, decaying with well-resolved rotational correlation times of 0.03, 0.2, and greater than or equal to 1 ms. Mild proteolytic removal of the water-soluble 41-kDa cytoplasmic domain of band 3 in ghosts results in a drastic increase in the fractional contributions of the two fastest depolarizing components. Our results, taken together with other data in the literature, imply that several classes of band 3 that differ greatly in mobility exist in ghosts and intact cells. The mobility of one class is hindered due to complexation with other membrane or cytoplasmic proteins mediated via the 41-kDa cytoplasmic domain. However, another class of band 3 molecules exists as homo-or heterooligomeric complexes larger than a dimer that are stabilized by hydrophobic interactions involving the intramembranal domain. Finally, the presence of the (previously undetected) 0.03-ms anisotropy component strongly suggests that a significant fraction of band 3 in both ghosts and intact cells is highly mobile and diffuses at the rate expected for a freely rotating dimer in the erythrocyte membrane.

Novel fluorogenic substrates for assaying retroviral proteases by resonance energy transfer

The 11-kD protease (PR) encoded by the human immunodeficiency virus 1 (HIV-1) is essential for the correct processing of viral polyproteins and the maturation of infectious virus, and is therefore a target for the design of selective acquired immunodeficiency syndrome (AIDS) therapeutics. To facilitate the identification of novel inhibitors of HIV-1 PR, as well as to permit detailed studies on the enzymology and inhibition of this enzyme, a continuous assay for its activity was developed that was based on intramolecular fluorescence resonance energy transfer (RET). The assay used the quenched fluorogenic substrate 4-(4-dimethylaminophenylazo)benzoic acid (DABCYL)--Ser Gln Asn Tyr Pro Ile Val Gln--5-[(2-aminoethyl)amino]naphthalene-1 sulfonic acid (EDANS), whose peptide sequence is derived from a natural processing site for HIV-1 PR. Incubation of recombinant HIV-1 PR with the fluorogenic substrate resulted in specific cleavage at the Tyr-Pro bond and a time-dependent increase in fluorescence intensity that was linearly related to the extent of substrate hydrolysis. An internally quenched fluorogenic substrate was also designed that was selectively cleaved by the related PR from avian myeloblastosis virus (AMV). The fluorescence quantum yields of the HIV-1 PR and AMV PR substrates in the RET assay increased by 40.0- and 34.4-fold, respectively, per mole of substrate cleaved. Because of its simplicity, rapidity, and precision in the determination of reaction rates required for kinetic analysis, this method offers many advantages over the commonly used high-performance liquid chromatography- or electrophoresis-based assays for peptide substrate hydrolysis by retroviral PRs.

The lack of relationship between fluorescence polarization and lateral diffusion in biological membranes

An investigation has been carried out of the relationship between changes in the fluorescence polarization of 1,6-diphenyl-1,3,5-hexatriene (DPH) and concomitant changes in the lateral diffusion of proteins and lipid probes in membranes. Plasma membranes from lymphocytes and a CH1 mouse lymphoma line were treated with up to 70 mol% (relative to the total membrane phospholipid) of oleic or linoleic fatty acids. Under these conditions the fluorescence polarization of DPH decreased by between 8 and 15% which, in the framework of the microviscosity approach, suggests a membrane fluidity change of between 20 and 50%. The lateral diffusion coefficients of surface immunoglobin and the lipid probes 3,3'-dioctadecylindocarbocyanine and pyrene were also measured in these membranes using the fluorescence photobleaching recovery technique and the rate of pyrene excimer formation. The diffusion rates were found to be unaffected by the presence of free fatty acids. Hence despite large 'microviscosity' changes as reported by depolarization of DPH fluorescence, lateral diffusion coefficients are essentially unchanged. This finding is consistent with the idea that perturbing agents such as free fatty acids do not cause a general fluidization of the membrane but act locally to alter, for example, protein function. It is also consistent with the suggestion that lateral mobility of membrane proteins is not modulated by the lipid viscosity.

Emission wavelength-dependent decay of the 9-anthroyloxy-fatty acid membrane probes

Using the phase-modulation technique, we have measured the fluorescence decay of 2- and 12-(9-anthroyloxy)-stearic acid (2- and 12-AS) and 16-(9-anthroyloxy)-palmitic acid (16-AP) bound to egg phosphatidylcholine vesicles or dissolved in nonpolar solvents. Heterogeneity analysis demonstrates that the decay is generally not monoexponential and exhibits large component variations across it emission spectrum. The mean decay time increases (and in parallel, the steady-state polarization decreases) monotonically with increasing wavelength from values at the blue end. The decay at the red side of the emission spectrum contains an exponential term with a negative amplitude, indicating that emission occurs from intermediates created in the excited-state. This behavior is interpreted as arising from intramolecular fluorophore relaxation occurring on the time scale of the fluorescence lifetime. We believe this to be the first study of wavelength-dependent fluorescent emission which is dominated by an intramolecular relaxation process. Although the three probes exhibit qualitatively similar effects, the emission band variations are greatest for 2-AS and smallest for 16-AP. The differences among the probes are not entirely due to environmental factors as demonstrated, for example, by the emission polarization differences observed in the isotropic solvent paraffin oil. In summary, while these findings point out some of the complexities in the 9-anthroyloxy-fatty acids as membrane probes, they also indicate how these complexities might be used as a sensitive measure of lipid-probe interaction.

Emission-wavelength-dependent decay of the fluorescent probe N-phenyl-1-naphthylamine

We have measured the fluorescence decay of N-phenyl-1-naphthylamine using the phase-modulation method, in several solvent systems and egg phosphatidylcholine vesicles. The decay is monoexponential in pure solvents (both polar and non-polar) of low viscosity. In polar viscous solvents or in non-polar solvents containing an added polar solute, the decay is heterogeneous and emission wavelength dependent. In such cases, dielectric relaxation and/or excited-rate complexing give rise to a shift of the emission spectrum on the nanosecond time scale. Emission-wavelength-dependent decay was also observed when N-phenyl-1-naphthylamine was bound to egg phosphatidylcholine vesicles. From these results as well as the position of the emission spectral maximum, we conclude that N-phenyl-1-naphthylamine probes the ester-carbonyl region of the phospholipid acyl chains, where it undergoes an excited-state reaction. This result contradicts the often made assumption that N-phenyl-1-naphthylamine probes the deeper hydrocarbon region of the bilayer.

Distribution of shape-changing compounds across the red cell membrane

The effects of two oppositely charged pyrene derivatives, 1-pyrenebutyrylcholine (PBC) and 1-pyrenebutyric acid (PBA), on red blood cell shape have been examined. Both compounds convert normal biconcave erythrocytes into echinocytes. However, with extended incubation time at elevated temperature, the morphology of PBC-induced echinocytes is reversed. Examination of probe uptake confirmed that, in contrast to PBA, equilibration of PBC with intact cells occurs very slowly. For PBA-induced echinocytes, it was possible to quantitate the fraction of probe bound in each half of the bilayer from nanosecond fluorescence measurements. Analysis of the heterogeneous decay showed that 71% of the bound PBA was associated with a lifetime (tau) of 102 ns and 29% with tau = 8 ns. It is likely that the later, highly quenched, component corresponds to fluorophores bound at the cytoplasmic surface because of efficient energy transfer to hemoglobin and that the long component corresponds to probe bound exclusively at the outer surface. Evidence in support of this interpretation was obtained by showing that when the paramagnetic cation Mn2+ bound at the extracellular surface the 102-ns component is quenched. The excimer fluorescence of PBC bound to red cells was examined and found to show time and temperature dependencies which correlate with morphological effects. These results indicate that red cells become crenated with PBC molecules are highly concentrated in the outer bilayer half and that shape reversal is subsequently brought about as PBC permeates and accumulates in the inner bilayer half. Finally, hemolysis protection due to PBC or PBA binding was observed also to show striking correlations with cell shape, In summary, these findings support the hypothesis [Sheetz, M. P., & Singer, S. J. (1974) Proc. Natl. Acad. Sci. U.S.A. 71, 4457] that shape changes are induced in red cells by amphiphilic molecules as a consequence of their relative partitioning between the two halves of the bilayer.