Heterodimerization between members of the Bcl-2 family of proteins is a key event in the regulation of programmed cell death. The molecular basis for heterodimer formation was investigated by determination of the solution structure of a complex between the survival protein Bcl-xL and the death-promoting region of the Bcl-2-related protein Bak. The structure and binding affinities of mutant Bak peptides indicate that the Bak peptide adopts an amphipathic alpha helix that interacts with Bcl-xL through hydrophobic and electrostatic interactions. Mutations in full-length Bak that disrupt either type of interaction inhibit the ability of Bak to heterodimerize with Bcl-xL.

THE Bcl-2 family of proteins regulate programmed cell death by an unknown mechanism. Here we describe the crystal and solution structures of a Bcl-2 family member, Bcl-xL (ref. 2). The structures consist of two central, primarily hydrophobic alpha-helices, which are surrounded by amphipathic helices. A 60-residue loop connecting helices alpha1 and alpha2 was found to be flexible and non-essential for anti-apoptotic activity. The three functionally important Bcl-2 homology regions (BH1, BH2 and BH3) are in close spatial proximity and form an elongated hydrophobic cleft that may represent the binding site for other Bcl-2 family members. The arrangement of the alpha-helices in Bcl-xL is reminiscent of the membrane translocation domain of bacterial toxins, in particular diphtheria toxin and the colicins. The structural similarity may provide a clue to the mechanism of action of the Bcl-2 family of proteins.

Amyloid b-peptide (Ab) 1)42 oligomers have recently been discussed as intermediate toxic species in Alzheimer's disease (AD) pathology. Here we describe a new and highly stable Ab 1)42 oligomer species which can easily be prepared in vitro and is present in the brains of patients with AD and Ab 1)42 -overproducing transgenic mice. Physicochemical characterization reveals a pure, highly water-soluble globular 60-kDa oligomer which we named 'Ab 1)42 globulomer'. Our data indicate that Ab 1)42 globulomer is a persistent structural entity formed independently of the fibrillar aggregation pathway. It is a potent antigen in mice and rabbits eliciting generation of Ab 1)42 globulomer-specific antibodies that do not cross-react with amyloid precursor protein, Ab 1)40 and Ab 1)42 monomers and Ab fibrils. Ab 1)42 globulomer binds specifically to dendritic processes of neurons but not glia in hippocampal cell cultures and completely blocks long-term potentiation in rat hippocampal slices. Our data suggest that Ab 1)42 globulomer represents a basic pathogenic structural principle also present to a minor extent in previously described oligomer preparations and that its formation is an early pathological event in AD. Selective neutralization of the Ab globulomer structure epitope is expected to have a high potential for treatment of AD. Keywords: Alzheimer's disease, amyloid b-peptide, hippocampal neurons, long-term potentiation, oligomers, polymerization. Abbreviations used: Ab, amyloid b-peptide; AD, Alzheimer's disease; ADDL, amyloid b-peptide-derived diffusible ligand; AFM, atomic force microscopy; APP, amyloid precursor protein; CHO, chinese hamster ovary; CL, cross-linked; CSF, cerebrospinal fluid; DAPI, 4¢,6-Diamidino-2-phenylindole; DIV, days in vitro; EPSPs, excitatory postsynaptic potentials; fEPSP, field excitatory postsynaptic potential; GFAP, glial fibrillary acidic protein; HFIP, 1,1,1,3,3,3 hexafluoro-2-propanol; HFS, high-frequency stimulation; LTP, long-term potentiation; MAP2, microtubule associated protein-2; NBT/BCIP, Nitro blue tetrazolium chloride/5-Bromo-4-chloro-3-indolyl phosphate; PAGE, polyacrylamide gel electrophoresis; PBS, phosphate-buffered saline; PSD-95, postsynaptic density protein 95; RT, room temperature; sAPPa, soluble amyloid precursor protein alpha; SDS, sodium dodecyl sulfate; TBS, Tris-buffered saline; TBST, 0.05% Tween 20 in Tris-buffered saline.

The pleckstrin homology (PH) domain is a new protein module of around 100 amino acids found in several proteins involved in signal transduction. Although its specific function has yet to be elucidated, the carboxy-terminal regions of many PH domains bind to the beta gamma subunits of G proteins. On the basis of structural similarities between PH domains and lipid-binding proteins, we have proposed that PH domains may be binding to lipophilic molecules. Indeed, many of the proteins that contain this domain associate with phospholipid membranes, and disruption of this domain can interfere with membrane association. Here we report that PH domains bind to phosphatidylinositol-4,5-bisphosphate and show that the lipid-binding site is located at the lip of the beta-barrel. This suggests that PH domains may be important for membrane localization of proteins through interactions with phosphatidylinositol-4,5-bisphosphate.