Membrane-induced folding of cecropin A

Lipid membranes manifest a diverse array of surface forces that can fold an d orient an approaching protein. To better understand these forces and thei r ability to influence protein function, we have used infrared spectroscopy with isotopic editing to characterize the 37-residue membrane-active antim icrobial polypeptide cecropin A as it approached, adsorbed onto, and finall y penetrated various lipid membranes. Intermediate stages in this process w ere isolated for study by the use of internal reflection and Langmuir troug h techniques. Results indicate that this peptide adopts well-ordered second ary structure while superficially adsorbed to a membrane surface. Its confo rmation is predominantly alpha-helical, although some beta structure is lik ely to be present. The longitudinal axis of the helical structure, and the transverse axes of any beta structure, are preferentially oriented parallel to the membrane surface. The peptide expands the membrane against pressure when it penetrates the membrane surface, but its structure and orientation do not change. These observations indicate that interactions between the p eptide and deeper hydrophobic regions of the membrane provide energy to per form thermodynamic work, but separate and distinct interactions between the peptide and superficial components of the membrane are responsible for pep tide folding. These results have broad implications for our understanding o f the mechanism of action and the specificity of these antimicrobial peptid es.