Proteolytic activity of 26S proteasomes isolated from muscles of the tobacco hornworm, Manduca sexta: Differences between surviving muscles and thoseundergoing developmentally programmed cell death

The intersegmental muscles (ISMs) of tobacco hornworm, Manduca sexta are a well-characterised model system for examining the biochemical changes that accompany programmed cell death during development. When the ISMs become co mmitted to die, there are dramatic increases in both the ubiquitin-expressi on, and ubiquitin-dependent proteolysis, Since the 26S proteasome is respon sible for ATP/ubiquitin-dependent proteolysis in cells, we examined its enz ymatic properties. Specific chymotrypsin-like proteolytic activity of 26S p roteasomes isolated from ISM is four times higher than that of surviving fl ight muscle (FM), However, specific activity does not change between develo pmental stages within ISM or FM. The difference between proteolytic capacit y of the two kinds of muscles is even higher when the ISM become committed to die because 26S proteasome content of ISM increases just before cell dea th. These observations underline the role of 26S proteasome in programmed c ell death.