Fucoidan, a sulfated fucopolysaccharide, mimics the fucosylated glycans of
glycoproteins and has therefore been used as a probe for investigating the
role of membrane polysaccharides in cell-cell adhesion. In the present repo
rt we have characterized the interaction of fucoidan with the Ca2+- and pho
spholipid-binding protein annexin II tetramer (AIIt). AIIt bound to fucoida
n with an apparent K-d of 1.24 +/- 0.69 nM (mean +/- SD, n = 3) with a stoi
chiometry of 0.010 +/- 0.001 mel of fucoidan/mol of AIIt (mean +/- SD, n 3)
. The binding of fucoidan to AIIt was Ca2+-independent. Furthermore, in the
presence but not the absence of Ca2+, the binding of fucoidan to AIIt caus
ed a decrease in the alpha-helical content from 32% to 7%. A peptide corres
ponding to a region of the p36 subunit of AIIt, F(306)-S(313), which contai
ns a Cardin-Weintraub consensus sequence for heparin binding, was shown to
undergo a conformational change upon fucoidan binding. This suggests that h
eparin and fucoidan bound to this region of AIIt. The binding of fucoidan b
ut not heparin by Ant also inhibited the ability of AIIt to bind to and agg
regate phospholipid liposomes. These results suggest that the binding of AI
It to the carbohydrate conjugates of certain membrane glycoproteins may hav
e profound effects on the structure and biological activity of AIIt.