8.B.9 The Triflin Toxin (Triflin or CRISP) Family
Triflin toxins, present in snake venom glands, block smooth muscle contraction elicited by high K+-induced depolarization, but not caffeine-stimulated contraction. It may target voltage-gated Ca2+ channels. The 3-d structure dimension at 2.4A resolution has been described (Shikamoto et al., 2005). A 39aa peptide, similar to the N-terminus of triflin, is the active Ca2+ blocker (Aoki et al., 2008). The 240 aa protein (with signal sequences) is homologous to proteases and protease inhibitiors. Another member of the CRISP or SCP family, helothemine, is a toxic peptide secreted by the beaded lizard. It blocks Ca2+-transporting rhanodine receptors.
SCP-like extracellular protein domains of the CRISP-like sub-family,include plant pathogenesis-related protein 1 (PR-1), CRISPs, mammalian cysteine-rich secretory proteins, which combine SCP with a C-terminal cysteine rich domain, and allergen 5 from vespid venom. Involvement of CRISP in response to pathogens, fertilization, and sperm maturation have been proposed. One member, Tex31 from the venom duct of Conus textile, has been shown to possess proteolytic activity sensitive to serine protease inhibitors. SCP has also been proposed to be a Ca+ chelating serine protease. The Ca+-chelating function would fit with various signaling processes that members of this family, such as the CRISPs, are involved in as supported by sequence and structural evidence of a conserved pocket containing two histidines and a glutamate. It also may explain how helothermine, a toxic peptide secreted by the beaded lizard, blocks Ca+transporting ryanodine receptors. One member, DE or CRISP-1, has been shown to mediate gamete fusion by binding to the egg surface; a sequence motif in the SCP domain plays a role in that binding (Sunagar et al. 2012).
Cycseine-rich venom protein, Triflin. Blocks contraction of smooth muscle elicited by high potassium-induced depolarization. May also target voltage-gated calcium channels (Cav) in smooth muscle cells. The crystal structure of triflin, a snake venom derived blocker of high K+-induced artery contraction has been solved at 2.4A resolution. Triflin consists of two domains. The first 163 residues form a large globular body with an αβα sandwich core which resembles pathogenesis-related proteins of group-1 (PR-1). Two glutamic acid-associated histidine residues are located in an elongated cleft. A Cd2+ resides in this binding site, and forms a five-coordination sphere. The subsequent cysteine-rich domain adopts a rod-like shape, which is stabilized by five disulfide bridges. Hydrophobic residues, which may obstruct the target ion channel, are exposed to the solvent. A concave surface, which is surrounded by these two domains, may play a role in the binding to the target receptor, leading to ion channel blockage. The C-terminal cysteine-rich region has a similar tertiary structure to voltage-gated potassium channel blocker toxins such as BgK and ShK (Shikamoto et al. 2005).
Triflin of Trimeresurus flavoviridis (Q8JI39)
Natrin of Naja atra (Q7T1K6)
Helothermine of 242 aas. Alters a variety of ion channel activities, including voltage-gated potassium channels (Kv), voltage-gated calcium channels (L-, N-, and P-type) (Cav) and ryanidine receptors (RyR) (Morrissette et al. 1995).
Helothemine of Mexican beaded lizard (Heloderma horridum horridum)
Cysteine-rich venom protein of 33 aas, Tripurin.
Tripurin of Trimeresurus purpureomaculatus (shore pit viper)
Basic Pathogenesis protein 1 of 177 aas, BPR1
BPR1 of Nicotiana tabacum (common tobacco)
Venom Allergen 5 of 254 aas
Alleergen 5 of Culex quinquefasciatus (Southern house mosquito) (Culex pungens)