In eukaryotes, the ubiquitin-proteasome machinery regulates a number of fundamental cellular processes through accurate and tightly controlled protein degradation pathways. We have, herein, examined the effects of proteasome functional disruption in Dmp53 (+/+) (wild-type) and Dmp53 (-/-) Drosophila melanogaster fly strains through utilization of Bortezomib, a proteasome-specific inhibitor. We report that proteasome inhibition drastically shortens fly life-span and impairs climbing performance, while it also causes larval lethality and activates developmentally irregular cell death programs during oogenesis. Interestingly, Dmp53 gene seems to play a role in fly longevity and climbing ability. Moreover, Bortezomib proved to induce endoplasmic reticulum (ER) stress that was able to result in the engagement of unfolded protein response (UPR) signaling pathway, as respectively indicated by fly Xbp1 activation and Ref(2)P-containing protein aggregate formation. Larva salivary gland and adult brain both underwent strong ER stress in response to Bortezomib, thus underscoring the detrimental role of proteasome inhibition in larval development and brain function. We also propose that the observed upregulation of autophagy operates as a protective mechanism to "counterbalance" Bortezomib-induced systemic toxicity, which is tightly associated, besides ER stress, with activation of apoptosis, mainly mediated by functional Drice caspase and deregulated dAkt kinase. The reduced life-span of exposed to Bortezomib flies overexpressing Atg1_RNAi or Atg18_RNAi supports the protective nature of autophagy against proteasome inhibition-induced stress. Our data reveal the in vivo significance of proteasome functional integrity as a major defensive system against cellular toxicity likely occurring during critical biological processes and morphogenetic courses.