MDMX promotes proteasomal turnover of p21 at G1 and early S phases independently of, but in cooperation with, MDM2.
We have shown previously that MDM2 promotes the degradation of the cyclin-dependent kinase inhibitor p21 through a ubiquitin-independent proteolytic pathway. Here we report that the MDM2 analog, MDMX, also displays a similar activity. MDMX directly bound to p21 and mediated its proteasomal degradation. Although the MDMX effect was independent of MDM2, they synergistically promoted p21 degradation when coexpressed in cells. This degradation appears to be mediated by the 26S proteasome, as MDMX and p21 bound to S2, one of the subunits of the 19S component of the 26S proteasome, in vivo. Conversely, knockdown of MDMX induced the level of endogenous p21 proteins that no longer cofractionated with 26S proteasome, resulting in G(1) arrest. The level of p21 was low at early S phase but markedly induced by knocking down either MDMX or MDM2 in human cells. Ablation of p21 rescued the G(1) arrest caused by double depletion of MDM2 and MDMX in p53-null cells. These results demonstrate that MDMX and MDM2 independently and cooperatively regulate the proteasome-mediated degradation of p21 at the G(1) and early S phases.
Jin
Yetao
Y
Department of Biochemistry and Molecular Biology, Indiana University School of Medicine, 635 Barnhill Dr., MS4053, Indianapolis, IN 46202, USA.
Zeng
Shelya X
SX
Sun
Xiao-Xin
XX
Lee
Hunjoo
H
Blattner
Christine
C
Xiao
Zhixiong
Z
Lu
Hua
H
D000818:Animals; D018797:Cell Cycle Proteins; D002460:Cell Line; D050759:Cyclin-Dependent Kinase Inhibitor p21 / Q000378:metabolism*; D016193:G1 Phase; D006207:Half-Life; D006801:Humans; D051379:Mice; D009687:Nuclear Proteins / Q000172:deficiency / Q000378:metabolism*; D046988:Proteasome Endopeptidase Complex / Q000378:metabolism*; D011485:Protein Binding; D021122:Protein Subunits / Q000378:metabolism; D011518:Proto-Oncogene Proteins / Q000172:deficiency / Q000378:metabolism*; D051736:Proto-Oncogene Proteins c-mdm2 / Q000172:deficiency / Q000378:metabolism*; D034741:RNA, Small Interfering / Q000378:metabolism; D016196:S Phase; D016159:Tumor Suppressor Protein p53 / Q000172:deficiency / Q000378:metabolism; D054875:Ubiquitination
D018797:Cell Cycle Proteins; D050759:Cyclin-Dependent Kinase Inhibitor p21; C482775:MDM4 protein, human; D009687:Nuclear Proteins; D021122:Protein Subunits; D011518:Proto-Oncogene Proteins; D034741:RNA, Small Interfering; D016159:Tumor Suppressor Protein p53; C497206:MDM2 protein, human; D051736:Proto-Oncogene Proteins c-mdm2; D046988:Proteasome Endopeptidase Complex