N4BP1 is a newly identified nucleolar protein that undergoes SUMO-regulated polyubiquitylation and proteasomal turnover at promyelocytic leukemia nuclear bodies.
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. Here, we describe Nedd4-mediated polyubiquitylation and proteasomal degradation of N4BP1. In addition, we show 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 have localized endogenous N4BP1 predominantly to the nucleolus in primary cells. However, a small fraction is found at promyelocytic leukemia (PML) nuclear bodies (NBs). In cells deficient for SENP1 or in wild-type cells treated with the proteasome inhibitor MG132, there is 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.
Laboratory of Protein Dynamics and Signaling, Center for Cancer Research, National Cancer Institute, NCI-Frederick, Frederick, MD 21702, USA.
Prashant
P
Sharma
Rodolfo
R
Murillas
Huafeng
H
Zhang
Michael R
MR
Kuehn
D000818:Animals; D002352:Carrier Proteins / Q000378:metabolism*; D002460:Cell Line; D002466:Cell Nucleolus / Q000378:metabolism; D003546:Cysteine Endopeptidases; D004622:Embryo, Mammalian / Q000166:cytology / Q000378:metabolism; D010450:Endopeptidases / Q000378:metabolism; D045586:Intranuclear Inclusion Bodies / Q000378:metabolism*; D015473:Leukemia, Promyelocytic, Acute / Q000378:metabolism*; D051379:Mice; D008954:Models, Biological; D009154:Mutation / Q000235:genetics; D009687:Nuclear Proteins / Q000378:metabolism*; D025821:Polyubiquitin / Q000378:metabolism*; D046988:Proteasome Endopeptidase Complex / Q000378:metabolism*; D061988:Proteasome Inhibitors; D055550:Protein Stability; D021381:Protein Transport; D025841:Small Ubiquitin-Related Modifier Proteins / Q000378:metabolism*; D054875:Ubiquitination
D002352:Carrier Proteins; C522898:N4BP1 protein, mouse; D009687:Nuclear Proteins; D061988:Proteasome Inhibitors; D025841:Small Ubiquitin-Related Modifier Proteins; D025821:Polyubiquitin; D010450:Endopeptidases; D003546:Cysteine Endopeptidases; C501974:Senp1 protein, mouse; D046988:Proteasome Endopeptidase Complex