Prothoracicotropic Hormone Elicits a Rapid, Developmentally Specific Synthesis of β Tubulin in an Insect Endocrine Gland

Abstract

The production of ecdysteroid (molting) hormones by the insect prothoracic gland is controlled primarily by a brain neuropeptide hormone, prothoracicotropic hormone (PTTH). PTTH stimulates also the specific synthesis of three proteins in the prothoracic glands of the tobacco hornworm Manduca sexta (Rybczynski and Gilbert, 1994). Here, one of these proteins, p50 is identified as a β tubulin. The ability of PTTH to stimulate β tubulin synthesis increased dramatically late on Day 3 of the 10-day fifth larval instar. At this time and later, β tubulin synthesis in response to PTTH in vitro could be detected in some prothoracic glands 5-10 min after the onset of stimulation, and newly synthesized β tubulin entered the microtubule pool within 12 min. Levels of β tubulin in the glands of fifth instar larvae, measured by immunoblot, changed in a tissue-specific manner that paralleled or presaged circulating ecdysteroid levels. The accumulation of β tubulin in PTTH-stimulated prothoracic glands resulted from increased transcription and translation and not from a decreased protein turnover rate. Pulse-chase experiments indicate that the newly synthesized β tubulin had a very short half-life in vitro (≈0.5 hr). Studies with cycloheximide and actinomycin D indicated that β tubulin synthesis and ecdysteroid synthesis are coregulated and that β tubulin synthesis is regulated in a unique manner relative to most other prothoracic gland proteins. We conclude that β tubulin levels may play an important role in ecdysteroidogenesis, perhaps by influencing the dynamics of microtubule-dependent secretion or inter-organelle movement of ecdysteroid precursors.