[Intellectual Contribution]

Identification and characterization of a novel carrier protein involved in ant chemical communication

Toshimasa Yamazaki1, Wataru Tsuchiya1, Zui Fujimoto1, Mitsuhiro Miyazawa2, Jun Ishibashi2
1Biomolecular Research Unit, 2Insect Mimetics Research Unit
[Abstract]
A new carrier protein involved in ant chemical communication was identified and characterized. The ligand-binding pocket of this protein was composed of a flexible β-structure that allowed it to bind and deliver a wide range of hydrophobic semiochemicals. The defined molecular mechanism of ligand recognition may help us to develop new tools for pest ant management.
[Keywords]
Keywords: worker ant, semiochemical, carrier protein, structure-based drug design

[Background]

Ants are eusocial insects that distribute tasks among individuals belonging to different castes. To fulfill caste-specific tasks, ants have developed a sophisticated system of chemical communication using sensory organs (sensilla) that detect molecules carrying task-specific information (semiochemicals). The antennae are the major chemosensory organs in ants. Once the semiochemicals have entered the sensillum through pores, they reach the aqueous sensillum lymph, which contains carrier proteins that bind the hydrophobic molecules and deliver them to various chemoreceptors residing in the membrane of the neuron’s dendrites (Fig. 1). Worker ants are responsible for various tasks that are required for colony maintenance. In their chemical communication, α-helical carrier proteins, odorant-binding proteins (OBPs) and chemosensory proteins (CSPs), which accumulate in the sensillum lymph in the antennae, play roles in transporting semiochemicals to chemoreceptors. However, the number of these carrier proteins is not sufficient to bind the large number of semiochemicals that are recognized by ants. It is therefore hypothesized that there are undefined carrier proteins involved in chemical communication in worker ants and some of them must possess a ligand-binding pocket that interacts with a variety of semiochemicals. Screening for worker-antenna-specific genes in the Japanese carpenter ant, Camponotus japonicas, enabled us to identify a novel carrier protein that is capable of delivering various hydrophobic semiochemicals to chemosensory receptor neurons.
[Results and Discussion]
  1. We identified a worker-antenna-specific gene that is involved in chemical communication of the Japanese carpenter ant, Camponotus japonicas. We named the protein encoded by this gene CjapNPC2 due to its high similarity to the Niemann-Pick type C2 (NPC2) protein which is an essential carrier protein for intracellular cholesterol transport in vertebrates including human. CjapNPC2 was exclusively expressed in the antennae and was specifically accumulated in the lymph-filled cavities of the basiconic sensilla.
  2. Ligand binding studies revealed that CjapNPC2 is able to bind various hydrophobic molecules including long-chain fatty acids, alcohols and acetates at neutral pH but not at an acidic pH, suggesting the pH-dependent ligand binding and dissociation that are characteristic features of the carrier protein. In addition, some of the tested ligands can provide electrophysiological signals in the antenna of worker ants. It is noteworthy that CjapNPC2 is not able to bind cholesterol while the vertebrate NPC2 does not bind to long-chain fatty acids (Fig. 2).
  3. The crystal structures of the apo and oleic acid-bound CjapNPC2 unveiled the molecular mechanism of the ligand recognition which distinct from those by the α-helical carrier proteins of OBP and CSP (Fig. 3). CjapNPC2 adopts a β-sandwich structure with a large hydrophobic cavity for binding of the ligand in a U-shaped conformation (Fig. 3A). Intrinsic flexibility of the ligand-binding cavity of CjapNPC2, particularly at the entrance regions, may contribute to its moderate selectivity and thus facilitate entry and binding of a wide range of potential semiochemicals.
[Future prospects]
  1. 1.As the function of CjapNPC2 is quite different from that known in vertebrates, this protein is an attractive target for development of new tools for pest ant management.
  2. The defined molecular mechanism of ligand recognition by CjapNPC2 should open the door to the structure-based design of safe ant insecticides that disrupt chemical communication.

Fig. 1. Schematic representation of the general structure of an ant olfactory hair. The ant chemosensory signal transduction pathway is initiated by transport of a semiochemical molecule in the sensillum lymph as a complex with its carrier protein to the proper chemoreceptor.

 

Fig. 2. Ligand selectivity of Niemann-Pick type c2 proteins (NPC2). CjapNPC2 from the Japanese carpenter ant can bind and deliver various potential semiochemicals including long-chain fatty acids, alcohols and acetates but not cholesterol. In contrast, vertebrate NPC2, does not bind long-chain fatty acids.

 

Fig. 3. Structures of semiochemical carrier proteins in insects. (A) Niemann-Pick type C2 protein from the Japanese carpenter ant (CjapNPC2) in complex with oleic acid. (B) Odrant-binding protein. (C) Chemosensory protein. Bound molecules are shown as space-filling models.

 

[Collaborators]

Yuko Ishida (Toyama Prefectural University), Takeshi Fujii, Yukio Ishikawa (University of Tokyo), Shigeru Matsuyama (University of Tsukuba)

[Reference]

  1. Ishida Y, Tsuchiya W, Fujii T, Fujimoto Z, Miyazawa M, Ishibashi J, Matsuyama S, Ishikawa Y, Yamazaki T (2014) Niemann–Pick type C2 protein mediating chemical communication in the worker ant Proceedings of the National Academy of Sciences of the United States of America 111 (10):3847-3852
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