Domain Organization of Membrane-Bound Factor VIII

Abstract

Factor VIII (FVIII) is the blood coagulation protein which when defective or deficient causes for hemophilia A, a severe hereditary bleeding disorder. Activated FVIII (FVIIIa) is the co-factor to the serine protease Factor IXa (FIXa) within the membrane-bound Tenase complex, responsible for amplifying its proteolytic activity more than 100,000 times, necessary for normal blood clotting. FVIII is composed of two non-covalently linked peptide chains: a light chain holding the membrane interaction sites and a heavy chain holding the main FIXa interaction sites. The interplay between the light and heavy chains in the membrane-bound state is critical for FVIII biological efficiency. Here, we present our cryo-electron microscopy and structure analysis studies of human FVIII light chain (LC), as helically assembled onto negatively charged single lipid bilayer nanotubes (LNT). The resolved FVIII-LC membrane-bound structure at 20 Å, supports aspects of our previously proposed FVIII structure from membrane-bound two-dimensional (2D) crystals, such as only the C2 domain interacts directly with the membrane [Stoilova-McPhie 2002]. The light chain is oriented differently in the FVIII membrane-bound helical and 2D crystal structures based on electron microscopy data and the 3D structure solved by X-ray [Ngo 2008]. The flexibility of the FVIII-LC domain organization in different crystal packing (3D, 2D and helical) is essential to understand the FVIII membrane-bound organization and its significance for hemostasis. SSM acknowledge Professor Ed Egelman from the University of Virginia at Charlotseville for help and support with the IHRSR algorithm and helical analysis. This work is supported by a National Scientist Development grant from the American Heart Association: 10SDG3500034 and UTMB start up funds to SSM. The Cryo-EM and initial structure analysis was carried out at the NCMI facility, courtesy of Professor Wah Chiu, supported by a National Center for Research Resources grant P41RR02250.