Fourier transform infrared linked analysis of conformational changes in annexin V upon membrane binding

Annexins are ubiquitous cellular proteins of unknown primary function that bind to anionic phospholipid membranes in a calcium-dependent manner. Correlative studies involving X-ray crystallography and electron microscopy suggest that annexins undergo a structural change upon binding to supported lipid monolayer membranes. In this investigation, novel spectroscopic and analytical techniques have been applied to verify and characterize this change. Soluble annexin V was examined with ordinary transmission infrared spectroscopy, while membrane-bound annexin V was examined with both transmission and internal reflection infrared spectroscopy. Spectra were processed by linked analysis, whereby multiple spectra are fit simultaneously with component bands that are constrained to share common fitting parameters. This approach is shown to enhance the sensitivity and accuracy of the bandfitting procedure. Our results are consistent with the general mode of membrane binding inferred from electron microscopy studies, and they provide independent support for the conclusion that annexin V undergoes a conformational change upon binding to lipid monolayer membranes. Most likely, this change involves the formation of new beta structure in which interstrand hydrogen bonds orient parallel to the membrane surface.