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Juvenile hormone (JH) is a key regulator of insect development and reproduction. In adult mosquitoes, it is essential for maturation of the ovary and normal male reproductive behavior, but how JH distribution and activity is regulated after secretion is unclear. Here, we report a new type of specific JH-binding protein, given the name mosquito juvenile hormone-binding protein (mJHBP), which circulates in the hemolymph of pupal and adult males and females. mJHBP is a member of the odorant-binding protein (OBP) family, and orthologs are present in the genomes of, , and mosquito species. Using isothermal titration calorimetry, we show that mJHBP specifically binds JH II and JH III but not eicosanoids or JH derivatives. mJHBP was crystallized in the presence of JH III and found to have a double OBP domain structure reminiscent of salivary "long" D7 proteins of mosquitoes. We observed that a single JH III molecule is contained in the N-terminal domain binding pocket that is closed in an apparent conformational change by a C-terminal domain-derived α-helix. The electron density for the ligand indicated a high occupancy of the natural 10 enantiomer of JH III. Of note, mJHBP is structurally unrelated to hemolymph JHBP from lepidopteran insects. A low level of expression of mJHBP in larvae suggests that it is primarily active during the adult stage where it could potentially influence the effects of JH on egg development, mating behavior, feeding, or other processes.

A 29-kDa nuclear juvenile hormone (JH)-binding protein from the epidermis of Manduca sexta larvae was purified by using the photoaflinity analog for JH II ([3H] Juvenile hormone (JH), a sesquiterpenoid, regulates both metamorphosis and reproduction in insects (1). In the larva, JH is present throughout the growth phase, and its presence at the time of the ecdysteroid-induced molt is critical to prevent metamorphosis. The primary morphogenetic effect of JH is to guide ecdysteroid action so as to prevent its activation of new programs resulting in a change of form. In the adult, by contrast, JH apparently acts alone to stimulate vitellogenin synthesis in the fat body or vitellogenin uptake into the oocyte. The fat body has high-affinity cytosolic and nuclear-binding proteins for JH, whereas in facilitating yolk uptake, JH apparently acts via a membrane receptor on the follicle cells (for review, see refs. 1 and 2).In the epidermis of tobacco hornworm (Manduca sexta) larvae, one-third of the JH that enters the cell is retained in the nucleus by two proteins (Kd = 7 and 88 nM) (3). Palli et al. (4), using tritiated photoaffinity analogs of JH, identified a 29-kDa nuclear protein and a 38-kDa cytosolic protein that specifically bound JH. The 29-kDa protein was present in the epidermis during the fourth-and fifth-instar growth phases but declined during the molt and disappeared at the time of pupal commitment of the epidermis (5). Its reappearance in both the larval and pupal epidermis after ecdysis depended on the presence of JH during the molt. Our paper reports the purification and subsequent cloning of the cDNA for this

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