Department of Biology, Stanford University, Stanford, United States
Marie-Therese Fischer
Department of Biology, Stanford University, Stanford, United States
Maria Dolores Moya Garzon
Sarafan ChEM-H, Stanford University, Stanford, United States; Wu Tsai Institute for Neuroscience, Stanford University, Stanford, United States; Department of Pathology, Stanford University, Stanford, United States
Alexandra E Rangel
Wu Tsai Human Performance Alliance, Stanford University, Stanford, United States
Elicio E Tapia
Department of Radiology, Stanford University, Stanford, United States
Julia T Tanzo
Sarafan ChEM-H, Stanford University, Stanford, United States; Wu Tsai Institute for Neuroscience, Stanford University, Stanford, United States
H Tom Soh
Wu Tsai Human Performance Alliance, Stanford University, Stanford, United States; Center for Taxonomy and Morphology, Leibniz Institute for the Analysis of Biodiversity Change, Hamburg, Germany; Department of Electrical Engineering, Stanford University, Stanford, United States
Luis A Coloma
Chan Zuckerberg Biohub, San Francisco, United States
Sarafan ChEM-H, Stanford University, Stanford, United States; Wu Tsai Institute for Neuroscience, Stanford University, Stanford, United States; Department of Pathology, Stanford University, Stanford, United States; Centro Jambatu de Investigación y Conservación de Anfibios, Fundación Jambatu, San Rafael, Ecuador
Department of Biology, Stanford University, Stanford, United States; Wu Tsai Institute for Neuroscience, Stanford University, Stanford, United States; Stanford Diabetes Research Center, Stanford University, Stanford, United States
Alkaloids are important bioactive molecules throughout the natural world, and in many animals they serve as a source of chemical defense against predation. Dendrobatid poison frogs bioaccumulate alkaloids from their diet to make themselves toxic or unpalatable to predators. Despite the proposed roles of plasma proteins as mediators of alkaloid trafficking and bioavailability, the responsible proteins have not been identified. We use chemical approaches to show that a ~50 kDa plasma protein is the principal alkaloid-binding molecule in blood of poison frogs. Proteomic and biochemical studies establish this plasma protein to be a liver-derived alkaloid-binding globulin (ABG) that is a member of the serine-protease inhibitor (serpin) family. In addition to alkaloid-binding activity, ABG sequesters and regulates the bioavailability of ‘free’ plasma alkaloids in vitro. Unexpectedly, ABG is not related to saxiphilin, albumin, or other known vitamin carriers, but instead exhibits sequence and structural homology to mammalian hormone carriers and amphibian biliverdin-binding proteins. ABG represents a new small molecule binding functionality in serpin proteins, a novel mechanism of plasma alkaloid transport in poison frogs, and more broadly points toward serpins acting as tunable scaffolds for small molecule binding and transport across different organisms.
