COORDINATED INDUCTION OF THE UBIQUITIN CONJUGATION PATHWAY ACCOMPANIES THE DEVELOPMENTALLY PROGRAMMED DEATH OF INSECT SKELETAL-MUSCLE

The developmentally programmed cell death of abdominal intersegmental muscles in the tobacco hawkmoth Manduca sexta is coincident with a 10- fold induction of the polyubiquitin gene as a hormonally regulated eve nt (Schwartz, L. M., Myer, A., Kosz, L., Engelstein, M., and Maier, C. (1990) Neuron 5, 411-419), Solid phase immunochemical assays measurin g intersegmental muscle pools of free and conjugated ubiquitin reveal that the induction of polyubiquitin mRNA is accompanied by a proportio nal increase in total ubiquitin polypeptide, Ubiquitin conjugate pools increase 10-fold at eclosion, during which loss of muscle protein mas s is maximum, A smaller but measurable increase in ubiquitin conjugate s is observed earlier in pupal development coincident with a modest en hanced degradation of myofibrillar proteins, Accumulation of ubiquitin conjugates is accompanied by induction in the pathway for polypeptide ligation, including the activating enzyme (E1), several carrier prote in (E2) isoforms, and ubiquitin:protein isopeptide ligase (E3), Both a ccumulation of ubiquitin polypeptide and the enzymes of the conjugatio n pathway are subject to regulation by declining titers of the insect molting hormone 20-hydroxyecdysone, which signals onset of programmed cell death in the intersegmental muscles, Thus, programmed cell death within the intersegmental muscles is accomplished in part by stimulati on of the ubiquitin-mediated degradative pathway through a coordinated induction of ubiquitin and the enzymes responsible for its conjugatio n to yield proteolytic intermediates, This suggests enzymes required f or ubiquitin conjugation may represent additional genes recruited for developmentally programmed death.