Membrane-induced folding and structure of membrane-bound annexin A1 N-terminal peptides - Implications for annexin-induced membrane aggregation

¹ Division of Molecular Biosciences, Department of Biological Sciences, Imperial College, London, UK
² Royal (Dick) School of Veterinary Studies, The University of Edinburgh, Easter Bush Veterinary Cent
³ Institut Laue-Langevin, 6, rue Jules Horowitz, BP 156, 38042 Grenoble Cedex 9, France
⁴ Neuroendocrine Unit, Dept of Cellular and Mol. Neurosci., Hammersmith Hospital, Imperial College
⁵ Structural Chemistry, Eskitis Institute for Cell & Molecular Therapies, Griffith University

^* To whom correspondence should be addressed. E-mail: a.hofmann{at}griffith.edu.au.

Submitted on August 13, 2007
Revised on September 7, 2007
Accepted on 22 October 2007

	Abstract

Annexins constitute a family of calcium-dependent membrane binding proteins, and can be classified into two groups, depending on the length of the N-terminal domain unique for each individual annexin. The N-terminal domain of annexin A1 can adopt an -helical conformation, and has been implicated in mediating the membrane aggregation behaviour of this protein. While the calcium-independent interaction of the annexin A1 N-terminal domain has been known for some time, there was no structural information about the membrane interaction of this secondary membrane binding site of annexin A1. The present study used circular diochroism spectroscopy to show that a rat annexin A1 N-terminal peptide possess random coil structure in aqueous buffer, but an -helical structure in the presence of small unilamellar vesicles. The binding of peptides to membranes was confirmed by surface pressure (Langmuir film balance) measurements using phosphatidylcholine/phosphatidylserine monolayers, which show a significant increase after injection of rat annexin A1 N-terminal peptides. Lamellar neutron diffraction with human and rat annexin A1 N-terminal peptides reveal an intercalation of the helical peptides with the phospholipid bilayer, with the helix axis lying parallel to the surface of membrane. Our findings confirm that phospholipid membranes assist the folding of the N-terminal peptides into -helical structures, and that this conformation enables favourable direct interactions with the membrane. The results are consistent with the hypothesis that the N-terminal domain of annexin A1 can serve as a secondary membrane binding site in the process of membrane aggregation by providing a peripheral membrane anchor.

Key Words: annexins, lamellar neutron diffraction, lateral surface pressure, membrane aggregation