Snake Venom and Receptor Crosslinking.

Abstract SCI-37

Nearly all poisonous snakes venoms contain proteins including snaclecs (snake C-type lectins) and metalloproteases, which influence haemostasis by interacting with coagulation factors or platelets. Like other venom constituents they weaken the prey and make it easier to swallow and digest. Although some of those that affect platelets inhibit receptors, particularly integrins, many activate platelets by clustering receptors. The main receptors involved are GPIb and GPVI and the snaclecs and metalloproteases activate platelets in the same way as their natural ligands, von Willebrand factor (VWF) and collagen. Both VWF and collagen are thought to activate platelets by clustering receptors acting in somewhat parallel ways. In both cases a mainly signalling receptor, GPIb or GPVI, respectively, is clustered to activate an integrin, aIIbb3 or a1b2, respectively. The first step can be simulated by agents that bind to and cross-link GPIb or GPVI. These include antibodies, synthetic peptides or snake venom proteins. Many of the activating snaclecs belong to the tetrameric heterodimer group such as flavocetin, convulxin and its homologues, and some use both receptors. There remain however many unanswered questions about the non-covalent structure of these molecules and how they interact with receptors on, possibly, more than one platelet to induce activation even at very low concentrations (pM). A second class of snaclecs, including aggretin (rhodocytin), which has CLEC-2 as receptor, and purpureotin that acts via GPIb, are non-covalent dimers of heterodimers but may reorganise to form higher non-covalent multimers. These non-covalent interactions are still poorly understood and may be responsible for the low activity of recombinant snaclecs. Several activating snaclecs are glycosylated. Although this may simply enhance solubility, interactions with prey molecules can not yet be excluded. Many simple heterodimeric snaclecs are superficially inhibitory of washed platelets but can be converted to activators by cross-linking, either naturally or artificially. Finally, a major problem in snake venom genomics is ascribing function, such as target molecules, to novel snaclecs identified by DNA sequence alone. Although modelling has made great progress there are still too many unknown factors.