Rpn4p is an extremely short-lived proteasome-associated protein that acts both as a positive and negative transcriptional regulator of the ubiquitin-proteasome system and as its substrate. The mechanisms of proteasomal degradation of Rpn4p have been studied in great detail; however, the mechanisms of its own action remain unclear and, first of all, its functional domains are unknown. To map the functionally important regions, a set of Rpn4p deletion derivates was constructed. The mutant proteins were expressed in Saccharomyces cerevisiae strain rpn4-Δ, their contents were determined by Western blotting, and their activity was assessed by measuring the mRNA levels of proteasome genes. Deletions of the C-terminal region, containing DNA-binding zinc finger domains, and the N-terminal region, having no homology to the transactivation domains of any known transcription factor, completely inactivated Rpn4p. Only one of the two acidic regions, putative transactivation domains, proved to participate in transcriptional activation. Deletions of the N-terminal region, NAD, or zinc finger domains rendered Rpn4p metabolically stable. These data provide new insights into the mechanisms of the Rpn4p functioning and degradation.