PROTHORACICOTROPIC HORMONE ELICITS A RAPID, DEVELOPMENTALLY SPECIFIC SYNTHESIS OF BETA-TUBULIN IN AN INSECT ENDOCRINE GLAND

The production of ecdysteroid (molting) hormones by the insect prothor acic gland is controlled primarily by a brain neuropeptide hormone, pr othoracicotropic hormone (PTTH). PTTH stimulates also the specific syn thesis of three proteins in the prothoracic glands of the tobacco horn worm Manduca sexta (Rybczynski and Gilbert, 1994). Here, one of these proteins, p50 is identified as a beta tubulin. The ability of PTTH to stimulate beta tubulin synthesis increased dramatically late on Day 3 of the 10-day fifth larval instar. At this time and later, beta tubuli n synthesis in response to PTTH in vitro could be detected in some pro thoracic glands 5-10 min after the onset of stimulation, and newly syn thesized beta tubulin entered the microtubule pool within 12 min. Leve ls of beta tubulin in the glands of fifth instar larvae, measured by i mmunoblot, changed in a tissue-specific manner that paralleled or pres aged circulating ecdysteroid levels. The accumulation of beta tubulin in PTTH-stimulated prothoracic glands resulted from increased transcri ption and translation and not from a decreased protein turnover rate. Pulse-chase experiments indicate that the newly synthesized beta tubul in had a very short half-life in vitro (approximate to 0.5 hr). Studie s with cycloheximide and actinomycin D indicated that beta tubulin syn thesis and ecdysteroid synthesis are coregulated and that beta tubulin synthesis is regulated in a unique manner relative to most other prot horacic gland proteins. We conclude that beta tubulin levels may play an important role in ecdysteroidogenesis, perhaps by influencing the d ynamics of microtubule-dependent secretion or interorganelle movement of ecdysteroid precursors. (C) 1995 Academic Press,Inc.