Structure, Function, and Evolution of Biogenic Amine-binding Proteins in Soft Ticks

Abstract

Two highly abundant lipocalins, monomine and monotonin, have been isolated from the salivary gland of the soft tick Argas monolakensis and shown to bind histamine and 5-hydroxytryptamine (5-HT), respectively. The crystal structures of monomine and a paralog of monotonin were determined in the presence of ligands to compare the determinants of ligand binding. Both the structures and binding measurements indicate that the proteins have a single binding site rather than the two sites previously described for the female-specific histaminebinding protein (FS-HBP), the histamine-binding lipocalin of the tick Rhipicephalus appendiculatus. The binding sites of monomine and monotonin are similar to the lower, low affinity site of FS-HBP. The interaction of the protein with the aliphatic amine group of the ligand is very similar for the all of the proteins, whereas specificity is determined by interactions with the aromatic portion of the ligand. Interestingly, protein interaction with the imidazole ring of histamine differs significantly between the low affinity binding site of FS-HBP and monomine, suggesting that histamine binding has evolved independently in the two lineages. From the conserved features of these proteins, a tick lipocalin biogenic amine-binding motif could be derived that was used to predict biogenic amine-binding function in other tick lipocalins. Heterologous expression of genes from salivary gland libraries led to the discovery of biogenic amine-binding proteins in soft (Ornithodoros) and hard (Ixodes) tick genera. The data generated were used to reconstruct the most probable evolutionary pathway for the evolution of biogenic amine-binding in tick lipocalins.Hemostasis (blood clotting, platelet aggregation, and vasoconstriction) and immunity (inflammation, pain, and adaptive immunity) are the primary ways that the vertebrate host can respond against parasitic invasion and subsequent blood loss caused by blood-feeding arthropods. Hematophagous arthropods modulate these defenses by secreting a pharmacological mixture of bioactive components in their saliva during feeding. Most blood-feeders will have at least an anti-clotting, antiplatelet, and vasodilatory component in their salivary arsenal (1).This classic paradigm of blood-feeding strategy can be extended to include anti-inflammatory proteins that scavenge biogenic amines, as their presence in all blood-feeding arthropods is becoming apparent. Thus far, scavengers of biogenic amines have evolved independently in assassin bugs, mosquitoes, and ticks, and this will probably hold for other bloodfeeders as well (2)(3)(4)(5). This is in part due to the myriad effects that biogenic amines have during host defenses against blood loss and parasite invasion.5-Hydroxytryptamine (5-HT) 2 (or serotonin) is secreted by platelets and thrombocytes, as well as mast cells of many mammals, and functions to increase vascular permeability as well as inducing platelet aggregation and vasoconstriction (6). Histamine is released by activated mast cells and basophils and...

Ancestral reconstruction of tick lineages

“…This will necessitate new methods of analysis and representation (Chan and Ragan, 2013). As an example of this, the tick lipocalin family had 1 member in 1993 (Keller et al 1993), 4 in 1999 (Paesen et al 1999), 5 in 2002(Sangamnatdej et al 2002), 20 in 2003(Mans et al 2003a), 38 in 2005(Mans, 2005, ~300 in 2008 (Mans et al 2008c), 584 added from the first next-generation sequencing project (Karim et al 2011) and 3867 in the current database. If current trends in the description of transcriptomes continue, in 20 years there will be more lipocalins in the database than proteins encoded in the genome of I.…”

“…In this regard, closely related proteins have been shown to possess different functions even if these functions have some similarity. As such, a general biogenic amine binding function (histamine or serotonin) may be assigned to lipocalins with the biogenic amine binding motif, but it is not possible to predict which molecule will be scavenged until empirically tested (Mans et al 2008c). Members of the moubatin clade may bind TXA 2 , LTB 4 or inhibit complement C5 and some of the molecular correlates of these functions have been elucidated and can be used for functional prediction .…”

“…Molecular modelling has even been used to predict potential binding partners for LTC 4 scavengers (Mans and Ribeiro, 2008c), while the canonical folds of the Kunitz-BPTI and lipocalin families have been successfully used to predict inhibitory or scavenging…”

“…This included studies of the BTSP family , the Kunitz/BPTI family (Dai et al 2012;Louw et al 2013;Mans and Neitz, 2004b;Mans et al , 2003b2002a;Schwarz et al 2014b), the lipocalin family (Beaufays et al 2008b;Mans, 2005;Mans and Neitz, 2004c;Ribeiro, 2008a, 2008b;Mans et al , 2008cMans et al , 2003a) and the apyrases (Stutzer et al 2009). These studies suggested that argasid BPTI anti-hemostatic factors share a common evolutionary pathway; that biogenic amine binding lipocalins and TXA 2 /LTB 4 scavenging lipocalins are closely related; that the biogenic amine binding site is probably ancestral; that tick lipocalins share a monophyletic origin with lipocalins from other metazoan and that apyrase activity probably evolved a single time in the ancestral tick lineage.…”

“…Functions that are conserved between argasids and ixodids include biogenic amine binding lipocalins (Mans et al 2008c), apyrase activity (Stutzer et al 2009) and metalloprotease activity . These are also found in a salivary gland transcriptome of N. namaqua (B.J.…”