To identify targets of ubiquitin mediated protein degradation playing key roles in normal development, and that might potentially contribute to neoplasia when dysregulated, we carried out a yeast two hybrid screen for interaction partners of the E3 ubiquitin ligase Nedd4. This screen identified N4BP1, a novel developmentally expressed protein. Previous work has shown that N4BP1 also interacts with the related E3 ligase, ITCH, but is not a substrate for ITCH mediated ubiquitination. Rather, N4BP1 binding to ITCH, negatively regulates ITCH E3 activity directed toward its substrates, including the p53 related tumor suppressor proteins p73 and p63, as well as c-Jun. These results suggest that N4BP1 may have a role in regulating tumor progression and the response of cancer cells to chemotherapy. A number of proteins can be conjugated with both ubiquitin and the small ubiquitin-related modifier (SUMO), with crosstalk between these two post-translational modifications serving to regulate protein function and stability. We previously identified N4BP1 as a substrate for monoubiquitylation by the E3 ubiquitin ligase Nedd4. We have now found that N4BP1 undergoes Nedd4-mediated polyubiquitylation and proteasomal degradation. In addition, we found that N4BP1 can be conjugated with SUMO1 and that this abrogates N4BP1 ubiquitylation. Consistent with this, endogenous N4BP1 is stabilized in primary embryonic fibroblasts from mutants of the desumoylating enzyme SENP1, which show increased steady-state sumoylation levels. We localized endogenous N4BP1 predominantly to the nucleolus in primary cells. However, a small fraction was found at promyelocytic leukemia (PML) nuclear bodies (NBs). In cells deficient for SENP1 or in wild-type cells treated with the proteasome inhibitor MG132, we found considerable accumulation of N4BP1 at PML NBs. These findings suggest a dynamic interaction between subnuclear compartments, and a role for post-translational modification by ubiquitin and SUMO in the regulation of nucleolar protein turnover.