Characterization of the heparin binding properties of annexin II tetramer

In this report, we have characterized the interaction of heparin with the Ca²⁺- and phospholipid-binding protein annexin II tetramer (AIIt). Analysis of the circular dichroism spectra demonstrated that the Ca²⁺- dependent binding of AIIt to heparin caused a large decrease in the α- helical content of AIIt from ~44 to 31%, a small decrease in the β-sheet content from ~27 to 24%, and an increase in the unordered structure from 20 to 29%. The binding of heparin also decreased the Ca²⁺ concentration required for a half-maximal conformational change in AIIt from 360 to 84 μM. AIIt bound to heparin with an apparent K(d) of 32 ± 6 nM (mean ± S.D., n = 3) and a stoichiometry of 11 ± 0.9 mol of AIIt/mol of heparin (mean ± S.D., n = 3). The binding of heparin to AIIt was specific as other sulfated polysaccharides did not elicit a conformational change in AIIt. A region of the p36 subunit of AIIt (Phe³⁰⁶-Ser³¹³) was found to contain a Cardin- Weintraub consensus sequence for glycosaminoglycan recognition. A peptide to this region underwent a conformational change upon heparin binding. Other annexins contained the Cardin-Weintraub consensus sequence, but did not undergo a substantial conformational change upon heparin binding.