No abstract

The 37-residue islet amyloid polypeptide (IAPP) is thought to play an important role in the pathogenesis of type II diabetes. Despite a growing body of evidence implicating membrane interaction in IAPP toxicity, the membrane-bound form has not yet been well characterized. Here we used circular dichroism (CD) and fluorescence spectroscopy to investigate the molecular details of the interaction of IAPP with lipid membranes of varying composition. In the presence of membranes containing negatively charged phosphatidylserine (PS), we observed significant acceleration in the formation of IAPP aggregates. This acceleration is strongly modulated by the PS concentration and ionic strength, and is also observed at physiologically relevant PS concentrations. CD spectra of IAPP obtained immediately after the addition of membranes containing PS revealed features characteristic of an alpha-helical conformation approximately approximately 15-19 residues in length. After a longer incubation with membranes, IAPP gave rise to CD spectra characteristic of a beta-sheet conformation. Taken together, our CD and fluorescence data indicate that conditions that promote weakly stable alpha-helical conformations may promote IAPP aggregation. The potential roles of IAPP-membrane interaction and the novel membrane-bound alpha-helical conformation in IAPP aggregation are discussed.

Human islet amyloid polypeptide (hIAPP) misfolding is thought to play an important role in the pathogenesis of type II diabetes mellitus. It has recently been shown that membranes can catalyze the misfolding of hIAPP via an ␣-helical intermediate of unknown structure. To better understand the mechanism of membrane-mediated misfolding, we used site-directed spin labeling and EPR spectroscopy to generate a three-dimensional structural model of this membrane-bound form. We find that hIAPP forms a single ␣-helix encompassing residues 9 -22. The helix is flanked by N-and C-terminal regions that do not take up a clearly detectable secondary structure and are less ordered. Residues 21 and 22 are located in a transitional region between the ␣-helical structure and C terminus and exhibit significant mobility. The ␣-helical structure presented here has important implications for membrane-mediated aggregation. Anchoring hIAPP to the membrane not only increases the local concentration but also reduces the encounter between peptides to essentially a two-dimensional process. It is significant to note that the ␣-helical membrane-bound form leaves much of an important amyloidogenic region of hIAPP (residues 20 -29) exposed for misfolding. Misfolding of this and other regions is likely further aided by the low dielectric environment near the membrane that is known to promote secondary structure formation. Based upon these considerations, a structural model for membrane-mediated aggregation is discussed.Protein misfolding and amyloid fibril formation are common characteristics of a number of debilitating human diseases, such as Alzheimer disease, Parkinson disease, and type II diabetes mellitus (1, 2). In type II diabetes mellitus, the primary amyloidogenic agent is human islet amyloid polypeptide (hIAPP), 2 a 37-residue peptide that is synthesized in pancreatic islet ␤-cells and co-secreted with insulin. A number of findings support the notion that hIAPP misfolding plays an important role in disease. Approximately 95% of all patients with type II diabetes mellitus have large extracellular deposits composed of fibrillar hIAPP (2, 3). In vitro studies have shown that hIAPP is toxic when exogenously added to cultured human islet ␤-cells (4 -6) and that overexpression of hIAPP in COS cells results in accumulation of peptide aggregates and cell death (7,8). Mouse and rat IAPP do not misfold and are not toxic to cultured cells, suggesting that misfolding is a prerequisite for IAPP toxicity. Furthermore, mice and rats do not naturally develop type II diabetes mellitus, but transgenic mice and rats that express hIAPP form amyloid deposits and exhibit signs of diabetes, especially when expression occurs in a background of obesity (9 -13).Like other amyloidogenic peptides and proteins, hIAPP misfolds via a nucleation-dependent aggregation pathway in which small oligomeric assemblies precede the formation of mature amyloid fibrils (14). Biological membranes play two important roles in this process (for review, see Ref. 15). First, membrane...

Hydropathy plot methods form a cornerstone of membrane protein research, especially in the early stages of biochemical and structural characterization. Membrane Protein Explorer (MPEx), described in this article, is a refined and versatile hydropathy-plot software tool for analyzing membrane protein sequences. MPEx is highly interactive and facilitates the characterization and identification of favorable protein transmembrane regions using experimentbased physical and biological hydrophobicity scales. Besides allowing the consequences of sequence mutations to be examined, it provides tools for aiding the design of membrane-active peptides. MPEx is freely available as a Java Web Start application from our web site at

Constitutive ␣-helical membrane proteins (MPs) 1 are assembled in membranes by means of a translocation/insertion process that involves the translocon complex (1). After release into the membrane's bilayer fabric, a MP resides stably in a thermodynamic free energy minimum (evidence reviewed in Refs. 2 and 3). This means that the prediction of MP structure from the amino acid sequence is fundamentally a problem of physical chemistry, albeit a complex one. Physical influences that shape MP structure include interactions of the polypeptide chains with water, each other, the bilayer hydrocarbon core, the bilayer interfaces, and cofactors (Fig. 1). Two recent reviews (3, 4) provide extensive discussions of the evolution, structure, and thermodynamic stability of MPs. Here we provide a distilled (and updated) overview that addresses four broad questions.

No abstract

Type 2 diabetes mellitus (T2DM) is characterized by an ∼60% deficit in β-cell mass, increased β-cell apoptosis, and islet amyloid derived from islet amyloid polypeptide (IAPP). Human IAPP (hIAPP) forms oligomers, leading to either amyloid fibrils or toxic oligomers in an aqueous solution in vitro. Either application of hIAPP on or overexpression of hIAPP in cells induces apoptosis. It remains controversial whether the fibrils or smaller toxic oligomers induce β-cell apoptosis. Rifampicin prevents hIAPP amyloid fibril formation and has been proposed as a potential target for prevention of T2DM. We examined the actions of rifampicin on hIAPP amyloid fibril and toxic oligomer formation as well as its ability to protect β-cells from either application of hIAPP or endogenous overexpression of hIAPP (transgenic rats and adenovirus-transduced β-cells). We report that rifampicin (Acocella G. Clin Pharmacokinet 3: 108–127, 1978) prevents hIAPP fibril formation, but not formation of toxic hIAPP oligomers (Bates G. Lancet 361: 1642–1644, 2003), and does not protect β-cells from apoptosis induced by either overexpression or application of hIAPP. These data emphasize that toxic hIAPP oligomers, rather than hIAPP fibrils, initiate β-cell apoptosis and that screening tools to identify inhibitors of amyloid fibril formation are likely to be less useful than those that identify inhibitors of toxic oligomer formation. Finally, rifampicin and related molecules do not appear to be useful as candidates for prevention of T2DM.

Increasing evidence suggests that the misfolding and deposition of IAPP plays an important role in the pathogenesis of type II, or non-insulin-dependent diabetes mellitus (T2DM). Membranes have been implicated in IAPP-dependent toxicity in several ways: Lipid membranes have been shown to promote the misfolding and aggregation of IAPP. Thus, potentially toxic forms of IAPP can be generated when IAPP interacts with cellular membranes. In addition, membranes have been implicated as the target of IAPP toxicity. IAPP has been shown to disrupt membrane integrity and to permeabilize membranes. Since disruption of cellular membranes is highly toxic, such a mechanism has been suggested to explain the observed IAPP toxicity. Here, we review IAPP-membrane interaction in the context of (1) catalyzing IAPP misfolding and (2) being a potential origin of IAPP toxicity.