STRUCTURAL-ANALYSIS OF JUNCTIONS FORMED BETWEEN LIPID-MEMBRANES AND SEVERAL ANNEXINS BY CRYOELECTRON MICROSCOPY

The (annexin II-p11)(2) tetramer has been proposed to participate in e xocytosis and several other members of the annexin superfamily have be en reported to aggregate liposomes in vitro. In this context, the Ca2-dependent binding of several annexins to chromaffin granules and lipo somes was investigated by cryo-electron microscopy. The Ca2+-dependent aggregation of lipid membranes by (annexin II-p11)(2) results from th e spontaneous self-organization of the protein into two-dimensional pl aques, which are visualized in projection as characteristic junctions. The junctions have a constant thickness of 210(+/-10) Angstrom and pr esent a symmetrical distribution of electron-dense material arranged i nto seven stripes. They were observed over a wide range of Ca2+ concen trations, down to 2 mu M. The molecular components corresponding to th e seven electron-dense stripes were assigned as follows: the two assoc iated membranes give rise to two outer stripes each and the three cent ral stripes correspond to the (annexin II-p11)(2) tetramer. Each annex in II molecule interacts with the outer lipid leaflet of one membrane, giving rise to one stripe, while the central stripe is due to the (p1 1)(2) dimer with which both annexin II molecules interact. Both annexi n II and annexin I also induced the Ca2+-dependent aggregation of lipo somes via junctions that lack the central (p11)(2) moiety and present only six high-density stripes. As expected, both annexin V and annexin III bind to liposomes without inducing their aggregation. (C) 1997 Ac ademic Press Limited.