The transcription factor Sp1 is important for the expression of many cellular genes. Previously, it was shown that reduced O-glycosylation of Sp1 is associated with increased proteasome susceptibility. Sp1 undergoes proteasome-dependent degradation in cells stressed with glucose deprivation and adenylate cyclase activation, and this process is blocked in cells treated with glucosamine. In this study, using a reconstituted in vitro system, we identified the principal structural determinant in Sp1 that targets Sp1 for proteasome-dependent degradation. We found by using deletion analysis that the N-terminal 54 amino acids of Sp1 is required for Sp1 degradation. This element can act as an independent processing signal by directing degradation of an unrelated protein. Recognition of this Sp1 element by the proteasome-dependent system is saturable, and ubiquitination of this element is not required for recognition. Time course experiments revealed that Sp1 degradation is a two-step process. First, a discrete endoproteolytic cleavage occurs downstream of the target region immediately C-terminal to Leu 56 . The Sp1 sequence C-terminal to the cleavage site is subsequently degraded, whereas the N-terminal peptide remains intact. The identification of this Sp1 degradation-targeting signal will facilitate the identification of the critical proteins involved in the control of Sp1 proteasome-dependent degradation and the role, of O-GlcNAc in this process.