HDMX and its homologue HDM2 are two essential proteins for the cell; after genotoxic stress, both are phosphorylated near to their RING domain, specifically at serine 403 and 395, respectively. Once phosphorylated, both can bind the p53 mRNA and enhance its translation; however, both recognize p53 protein and provoke its degradation under normal conditions. HDM2 has been well-recognized as an E3 ubiquitin ligase, whereas it has been reported that even with the high similarity between the RING domains of the two homologs, HDMX does not have the E3 ligase activity. Despite this, HDMX is needed for the proper p53 poly-ubiquitination. Phosphorylation at serine 395 changes the conformation of HDM2, helping to explain the switch in its activity, but no information on HDMX has been published. Here, we study the conformation of HDMX and its phospho-mimetic mutant S403D, investigate its E3 ligase activity and dissect its binding with p53. We show that phospho-mutation does not change the conformation of the protein, but HDMX is indeed an E3 ubiquitin ligase in vitro; however, in vivo, no activity was found. We speculated that HDMX is regulated by induced fit, being able to switch activity accordingly to the specific partner as p53 protein, p53 mRNA or HDM2. Our results aim to contribute to the elucidation of the contribution of the HDMX to p53 regulation.

Catedra CONACyT Laboratorio de Biomarcadores Moleculares Instituto de Física Universidad Autónoma de San Luis Potosí México City México

Department of Medical Biosciences Umeå University SE 90185 Umeå Sweden

Équipe Labellisée Ligue Contre le Cancer Université Paris 7 INSERM UMR 1162 27 Rue Juliette Dodu 75010 Paris France

Laboratorio de Interacciones Biomoleculares y cáncer Instituto de Física Universidad Autónoma de San Luis Potosí Av Parque Chapultepec 1570 Privadas del pedregal 78210 SLP México

Regional Centre for Applied Molecular Oncology Masaryk Memorial Cancer Institute Zluty kopec 7 656 53 Brno Czech Republic

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