Project Summary/Abstract β-Barrels proteins exist in the outer membranes of Gram-negative bacteria, mitochondria, and chloroplasts. These proteins are composed of a sequence of covalently connected β-hairpins. Here, we propose to study the interactions of β-hairpin peptides with membranes, namely, binding, folding, insertion, and self-association. In this process, we will develop a foundation to understand the early steps of the interactions of β-barrel proteins in membranes, leading to the test of formation of multimeric barrels from β-hairpins. In the long-term we seek to develop methods for the design and folding of modular β-barrel proteins in membranes. The model we have in mind is a small membrane β-barrel protein, which consists of four β-hairpins in one polypeptide chain (eight strands). In this project specifically, we propose to design shorter peptides, of about 20-30 residues, that form β-hairpins, and to study their incorporation in synthetic lipid membranes. We also plan to understand the rules that determine proper membrane insertion and assembly of these peptides. The specific aims are to study the thermodynamics and kinetics of binding of β- hairpins to membranes, their structure in water and in the membrane, their insertion and orientation, pore formation, and the effect of lipid composition. Critical to these objectives is to understand how the peptide sequence determines binding, insertion, and folding. These studies are best performed in simple and easily controlled model systems, using biophysical methods with designed peptides and model lipid bilayer membranes. The study of folding of β-barrel membrane proteins, like that of membrane helical proteins, is complicated mainly because of the unfolded state of the protein in solution, where the nonpolar (hydrophobic) residues are exposed to water. Therefore, these proteins aggregate when unfolded in water. One approach to keep the protein monomeric in aqueous solution is to use detergents or denaturants. Instead, the idea here is to use small units—hairpins—that remain soluble because they are small. This allows us to study the entire process, from binding from aqueous solution to insertion, under equilibrium conditions, without co-solvent additives. A designed modular β-barrel would open the opportunities for applications enormously. The long-term idea is to engineer binding sites for small molecules, such as drugs, metabolites, or metal ions, thereby adding and a function to these modular β-barrels, with a large health- related potential set of applications. The modular nature will also allow the design of heteromultimeric β-barrels, by combination of different hairpins. Surmounting cellular membrane barriers, including intracellular compartments, is a major difficulty in the use of antibiotics, in drug delivery, and in molecular therapy. This proposal addresses the initial steps to the mechanisms required for a successful approach.

Project Narrative This project seeks to develop a fundamental understanding of interactions of β-peptides with membranes, to understand folding of β-barrel proteins, and to enable design of membrane proteins that can be assembled from modular elements. In the long term, this would enable us to design proteins with modified functionalities that can be added from the outside and assemble in the cell membrane. Those functions can improve uptake of drugs, antibiotics or nutrients (metabolites) at the cellular level, with vast applications to human health.