W0043

Structural studies of annexin II and related complexes. Barry Phipps, Stephen Litster, Hyoung-Min Kang, David Waisman and Penelope Codding^*, Departments of Biological Sciences and Medical Biochemistry, University of Calgary, Alberta, T2N 1N4 Canada, ^*University of Victoria, British Columbia V8W 2Y2 Canada

Annexins are a family of proteins which bind calcium ions and acidic phospholipids and possess a highly homologous C-terminal core domain known as the annexin fold. The N-terminal region of each annexin is unique. The biological function of the annexins is not well defined, however there is increasing evidence that they may be involved in processes such as anticoagulation, antithrombosis, mitogenesis and membrane trafficking. Annexin II is unique among the annexin family, existing as a monomer (AIIm, 36kDa), or a heterotetramer (AIIt), the latter consisting of two AIIm subunits and two p11 (11kDa) subunits. AIIm and AIIt are thought to play a role in exocytosis, endocytosis, and cell-cell adhesion. The structure of an N-terminally truncated AIIm has previously been reported (1). Here we report the results of our ongoing attempts to determine crystal structures of full-length AIIm and related complexes. X-ray diffraction data on two AIIm-heparin complexes has been collected to 2.2 and 1.9 Å resolution. Both were found to contain the N-terminally truncated form of AIIm; as yet, heparin has not been located, probably due to inherent flexibility. The structure obtained from crystals grown under reducing conditions does not contain the previously described (1) disulphide bridge between domains 2 and 3 of the C-terminal core. This arrangement may more accurately represent the intracellular protein form. The structure contains a previously unseen fifth calcium ion binding site. Recently, crystals of the heterotetramer have been obtained and data collected to 2.9 Å resolution.

1. Burger, A., et al.. (1996) J. Mol. Biol. 247, 839-847.

Work supported by the Alberta Heritage Foundation for Medical Research and the Medical Research Council of Canada.