Molecular basis of juvenile hormone signaling

• bHLH-PAS proteins Met and Tai form a JH receptor that mediates JH function in vivo.
• The Met–Tai receptor complex is a JH-activated transcription factor.
• JH activates Met and Tai via parallel, membrane-based second-messenger signaling.
• Biological roles of Met have been allotted to duplicated genes in some insects.
• JH signaling is dispensable for maintaining juvenile status in early-instar larvae.

Despite important roles played by juvenile hormone (JH) in insects, the mechanisms underlying its action were until recently unknown. A breakthrough has been the demonstration that the bHLH-PAS protein Met is an intracellular receptor for JH. Binding of JH to Met triggers dimerization of Met with its partner protein Tai, and the resulting complex induces transcription of target genes. In addition, JH can potentiate this response by phosphorylating Met and Tai via cell membrane, second-messenger signaling. An important gene induced by the JH–Met–Tai complex is Kr-h1, which inhibits metamorphosis. Kr-h1 represses an ‘adult specifier’ gene E93. The action of this JH-activated pathway in maintaining the juvenile status is dispensable during early postembryonic development when larvae/nymphs lack competence to metamorphose.