Crystal structure at 1.7 A of aggretin, a heterodimeric C-type lectin snake venom protein

Abstract number: P1761

Papagrigoriou^* E., Nadvaev† A., Clemetson‡ K. J., Emsley^* J.

†Theodor Kocher Institute, Switzerland; ‡Theodor Kocher, Switzerland ^*University of Leicester, UK;

Aggretin is a snake venom protein purified from Calloselasma rhodostoma. It belongs to the C-type lectin family of proteins and is a potent platelet activation inducer. Although extensively studied, aggretin's mechanism of action is not yet fully understood. Finding out how aggretin works is very important in deciphering the processes behind platelet activation. As a step in this direction we present the crystal structure of aggretin at 1.7 Å. The space group of the crystals is P21212 with the cell dimensions a = 64.32 b = 91.31 c = 118.92. The phases were calculated using molecular replacement with botrocetin as a search model. The structure of aggretin reveals extensive similarities in folding pattern to other members of the C-type lectin family. However, it also reveals marked differences which highlight aggretin's alternative mode of action. The X-ray data show that although the basic molecule of aggretin is a heterodimer of a and b subunits linked together by a disulfide bond, two of these heterodimers are linked together by noncovalent bonds, in a back-to-back, alpha to beta manner, forming a tetramer as active structure. It has been shown that aggretin binds to both GPIb and a2b1 on the platelet surface, though other receptors may also participate. Thus, each molecule of aggretin can bind at least two GPIb and two a2b1 molecules at the same time and cluster them. These data explain the ability of aggretin to activate and aggregate platelets, and also support receptor clustering as a primary mechanism involved in platelet activation.