Exaptation of two ancient immune proteins into a new dimeric pore-forming toxin in snails

GIGLIO, M.L.; ITUARTE, S.; MILESI, V.; DREON, M.S.; BROLA, T.R.; CARAMELO, J.; IP, J.C.H.; MATÉ, S.; QIU, J.W.; OTERO, L.H.; HERAS, H.

JOURNAL OF STRUCTURAL BIOLOGY

ACADEMIC PRESS INC ELSEVIER SCIENCE

Año: 2020 vol. 211

1047-8477

The Membrane Attack Complex-Perforin (MACPF) family is ubiquitously found in all kingdoms. They havediverse cellular roles, however MACPFs with pore-forming toxic function in venoms and poisons are very rare inanimals. Here we present the structure of PmPV2, a MACPF toxin from the poisonous apple snail eggs, that canaffect the digestive and nervous systems of potential predators. We report the three-dimensional structure ofPmPV2, at 17.2 Å resolution determined by negative-stain electron microscopy and its solution structure bysmall angle X-ray scattering (SAXS). We found that PV2s differ from nearly all MACPFs in two respects: it is adimer in solution and protomers combine two immune proteins into an AB toxin. The MACPF chain is linked by asingle disulfide bond to a tachylectin chain, and two heterodimers are arranged head-to-tail by non-covalentforces in the native protein. MACPF domain is fused with a putative new Ct-accessory domain exclusive toinvertebrates. The tachylectin is a six-bladed β-propeller, similar to animal tectonins. We experimentally validatedthe predicted functions of both subunits and demonstrated for the first time that PV2s are true poreformingtoxins. The tachylectin ?B? delivery subunit would bind to target membranes, and then the MACPF ?A?toxic subunit would disrupt lipid bilayers forming large pores altering the plasma membrane conductance. Theseresults indicate that PV2s toxicity evolved by linking two immune proteins where their combined preexistingfunctions gave rise to a new toxic entity with a novel role in defense against predation. This structure is anunparalleled example of protein exaptation.