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Structure of Human Annexin A6 at the Air-Water Interface and in a Membrane-Bound State

Marcin Golczak ^*, Aneta Kirilenko ^*, Joanna Bandorowicz-Pikula ^*, Bernard Desbat  and Slawomir Pikula ^*

^* Department of Cellular Biochemistry, Nencki Institute of Experimental Biology, Warsaw, Poland; and Laboratoire de Physicochimie Molèculaire, Unité Mixte de Recherche 5803, Université de Bordeaux I, Talence, France

Correspondence: Address reprint requests to Slawomir Pikula, Dept. of Cellular Biochemistry, Nencki Institute of Experimental Biology, 3 Pasteur St., 02-093 Warsaw, Poland. Tel.: 48-22-589-2347; Fax: 48-22-822-53-42; E-mail: s.pikula{at}nencki.gov.pl.

We postulate the existence of a pH-sensitive domain in annexin A6 (AnxA6), on the basis of our observation of pH-dependent conformational and orientation changes of this protein and its N- (AnxA6a) and C-terminal (AnxA6b) halves in the presence of lipids. Brewster angle microscopy shows that AnxA6, AnxA6a, and AnxA6b in the absence of lipids accumulate at the air-water interface and form a stable, homogeneous layer at pH below 6.0. Under these conditions polarization modulation IR absorption spectroscopy reveals significant conformational changes of AnxA6a whereas AnxA6b preserves its -helical structure. The orientation of protein -helices is parallel with respect to the interface. In the presence of lipids, polarization modulation IR reflection absorption spectroscopy experiments suggest that AnxA6a incorporates into the lipid/air interface, whereas AnxA6b is adsorbed under the lipid monolayer. In this case AnxA6a regains its -helical structures. At a higher pressure of the lipid monolayer the average orientation of the -helices of AnxA6a changes from flat to tilted by 45° with respect to normal to the membrane interface. For AnxA6b no such changes are detected, even at a high pressure of the lipid monolayer—suggesting that the putative pH-sensitive domain of AnxA6 is localized in the N-terminal half of the protein.

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