Gramicidin is an excellent model system for studying the passage of ions through biological membranes. The conformation of gramicidin is well deÐned in many di †erent solvent and lipid systems, as are its conductance and spectroscopic properties. It is a polymorphic molecule that can adopt two di †erent types of structure, the double helical "" pore ÏÏ and the helical dimer "" channel ÏÏ. This study investigated the inÑuence of the acyl chain length of membrane phospholipids on the conformations adopted by gramicidin. We used circular dichroism spectroscopy to examine the conformational equilibrium between the pore and channel forms in small unilamellar vesicles of phosphatidylcholine with acyl chain lengths of 18, 20 and 22 carbons. Our results show that in and C 18C 20 lipids almost all the gramicidin is in the channel form, while in the longer lipids the C 22 equilibrium shifts in favour of pore conformations, such that they form up to 43% of the total population. This change is attributed to the ability of the double helical conformation to tolerate more hydrophobic mismatch than the helical dimer, perhaps due to the greater number of stabilising intermolecular hydrogen bonds.

Antiamoebin I is a membrane-active peptaibol produced by fungi of the species Emericellopsis which is capable of forming ion channels in membranes. Previous structure determinations by x-ray crystallography have shown the molecule is mostly helical, with a deep bend in the center of the polypeptide, and that the backbone structure is independent of the solvent used for crystallization. In this study, the solution structure of antiamoebin was determined by NMR spectroscopy in methanol, a solvent from which one of the crystal structures was determined. The ensemble of structures produced exhibit a right-handed helical C terminus and a left-handed helical conformation toward the N-terminus, in contrast to the completely right-handed helices found in the crystal structures. The NMR results also suggest that a "hinge" region exists, which gives flexibility to the polypeptide in the central region, and which could have functional implications for the membrane insertion process. A model for the membrane insertion and assembly process is proposed based on the antiamoebin solution and crystal structures, and is contrasted with the assembly and insertion mechanism proposed for other ion channel-forming polypeptides.

The members of the APP gene family, consisting of the amyloid precursor protein and the amyloid precursor protein-like proteins I and 2 (APLPII-2) in different species all show a conserved, characteristic cysteine spacing in their N-terminal domain. This cysteine-rich domain is known as an interaction partner of apolipoprotein E (APP) and the prion protein (APLPI). It has a functional role in neurite outgrowth promotion and was found to be involved in the regulation of platelet activation and G-protein coupled signal transduction.The recombinant fragment APPl8-350 containing the cysteine-nch domain, the acidic and the KPI domains of APP was used to determine the disulphide bridge stmcture of APP. The fragment was cloned for secreted expression using the pPlC9 vector of the P&iu pusmris system. The overexpressed protein was purified under native conditions and then characterized. The cysteine-rich domain consists of two subdomains. The N-terminal subdomain shows structural similarity to a wide range of growth factors (Rossjohn el al.,

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