No abstract

No abstract

Fibrillar protein deposits (amyloid) in the pancreatic islets of Langerhans are thought to be involved in death of the insulinproducing islet ␤ cells in type 2 diabetes mellitus. It has been suggested that the mechanism of this ␤ cell death involves membrane disruption by human islet amyloid polypeptide (hIAPP), the major constituent of islet amyloid. However, the molecular mechanism of hIAPP-induced membrane disruption is not known. Here, we propose a hypothesis that growth of hIAPP fibrils at the membrane causes membrane damage. We studied the kinetics of hIAPP-induced membrane damage in relation to hIAPP fibril growth and found that the kinetic profile of hIAPP-induced membrane damage is characterized by a lag phase and a sigmoidal transition, which matches the kinetic profile of hIAPP fibril growth. The observation that seeding accelerates membrane damage supports the hypothesis. In addition, variables that are well known to affect hIAPP fibril formation, i.e., the presence of a fibril formation inhibitor, hIAPP concentration, and lipid composition, were found to have the same effect on hIAPP-induced membrane damage. Furthermore, electron microscopy analysis showed that hIAPP fibrils line the surface of distorted phospholipid vesicles, in agreement with the notion that hIAPP fibril growth at the membrane and membrane damage are physically connected. Together, these observations point toward a mechanism in which growth of hIAPP fibrils, rather than a particular hIAPP species, is responsible for the observed membrane damage. This hypothesis provides an additional mechanism next to the previously proposed role of oligomers as the main cytotoxic species of amyloidogenic proteins.amylin ͉ amyloid cytotoxicity ͉ large unilamellar vesicles ͉ protein-membrane interaction ͉ type 2 diabetes mellitus T ype 2 diabetes mellitus (DM2) is characterized histopathologically by the presence of fibrillar amyloid deposits in the pancreatic islets of Langerhans. Amyloid cytotoxicity is thought to be an early mechanism involved in death of insulin-producing islet ␤ cells in DM2 (1). The main component of islet amyloid, and the actual fibril-forming molecule, is a 37-amino acid peptide called human islet amyloid polypeptide (hIAPP) or amylin, which is produced together with insulin in the pancreatic islet ␤-cells. It is thought that ␤ cells of DM2 patients are somehow killed through hIAPP-induced damage of the ␤ cell membrane (2). However, our knowledge of the mechanism of hIAPP-induced membrane damage is extremely sparse. It is not known how cytotoxic hIAPP species interact with cellular membranes and induce cell death. Furthermore, it is not established whether cytotoxic hIAPP species are formed before contacting the membrane or whether a membrane environment is in fact required for the formation of cytotoxic hIAPP species.The prevailing view is that membrane damage and concomitant ␤ cell death are caused by cytotoxic hIAPP oligomers (2-9). There are indications that these oligomers form ion channels (2, 3), as has been suggeste...

We used solid-state deuterium NMR spectroscopy and an approach involving geometric analysis of labeled alanines (GALA method) to examine the structure and orientation of a designed synthetic hydrophobic, membrane-spanning alpha-helical peptide in phosphatidylcholine (PC) bilayers. The 19-amino-acid peptide consists of an alternating leucine and alanine core, flanked by tryptophans that serve as interfacial anchors: acetyl-GWW(LA)(6)LWWA-ethanolamine (WALP19). A single deuterium-labeled alanine was introduced at different positions within the peptide. Peptides were incorporated in oriented bilayers of dilauroyl- (di-C12:0-), dimyristoyl- (di-C14:0-), or dioleoyl- (di-C18:1(c)-) phosphatidylcholine. The NMR data fit well to a WALP19 orientation characterized by a distinctly nonzero tilt, approximately 4 degrees from the membrane normal, and rapid reorientation about the membrane normal in all three lipids. Although the orientation of WALP19 varies slightly in the different lipids, hydrophobic mismatch does not seem to be the dominant factor causing the tilt. We suggest rather that the peptide itself has an inherently preferred tilted orientation, possibly related to peptide surface characteristics or the disposition of tryptophan indole anchors relative to the lipids, the peptide backbone, and the membrane/water interface. Additionally, the data allow us to define more precisely the local alanine geometry in this membrane-spanning alpha-helix.

Specific interactions of membrane proteins with the membrane interfacial region potentially define protein position with respect to the lipid environment. We investigated the proposed roles of tryptophan and lysine side chains as "anchoring" residues of transmembrane proteins. Model systems were employed, consisting of phosphatidylcholine lipids and hydrophobic ␣-helical peptides, flanked either by tryptophans or lysines. Peptides were incorporated in bilayers of different thickness, and effects on lipid structure were analyzed. Induction of nonbilayer phases and also increases in bilayer thickness were observed that could be explained by a tendency of Trp as well as Lys residues to maintain interactions with the interfacial region. However, effects of the two peptides were remarkably different, indicating affinities of Trp and Lys for different sites at the interface. Our data support a model in which the Trp side chain has a specific affinity for a well defined site near the lipid carbonyl region, while the lysine side chain prefers to be located closer to the aqueous phase, near the lipid phosphate group. The information obtained in this study may further our understanding of the architecture of transmembrane proteins and may prove useful for refining prediction methods for transmembrane segments.In biological membranes, a variety of interactions can occur between lipids and proteins that affect protein as well as lipid properties and in which both the hydrophobic membrane core and the more polar membrane interfaces can be involved (1-3). Membrane proteins are able to span the lipid bilayer through interactions of their exposed hydrophobic segments with the lipid hydrocarbon acyl chains. In general, the length of these hydrophobic segments will approximately match the membrane hydrophobic thickness. However, also a mismatch between protein hydrophobic length and membrane hydrophobic thickness may occur. Such a mismatch can have considerable influence on membrane structure and function (reviewed in Ref. 4) and may, for example, be involved in protein sorting, microdomain formation, changes in protein activity, or changes in lipid structure and organization.In contrast to the hydrophobic core of a membrane, the membrane interface presents a complex and heterogeneous chemical environment, which accounts for a relatively large proportion of the total bilayer thickness (3). Specific interactions of membrane proteins with the interfacial region of the lipids may influence many functional processes, such as for instance membrane protein assembly, topology of membrane proteins, the mode of protein insertion into the membrane, and protein anchoring to the membrane. In addition, such interactions may play a determining role in hydrophobic mismatch (4).Analyses of the structure of transmembrane proteins suggest that two types of amino acids may be of special importance for interactions of membrane proteins with the interfacial region: aromatic amino acids, in particular tryptophans, which are enriched at both ends of tr...