Inhibits p53/TP53- and TP73/p73-mediated cell cycle arrest and apoptosis by binding its transcriptional activation domain. Inhibits degradation of MDM2. Can reverse MDM2-targeted degradation of TP53 while maintaining suppression of TP53 transactivation and apoptotic functions. The short isoform is a more potent inhibitor of TP53 activity than the long isoform. Found in a trimeric complex with USP2, MDM2 and MDM4. Interacts with USP2. Interacts (phosphorylated) with YWHAG; negatively regulates MDM4 activity toward TP53 (By similarity). Interacts with MDM2, TP53 and TP73. In all tissues tested. The short isoform is expressed in a variety of transformed cell lines. Region I is sufficient for binding TP53 and inhibiting its G1 arrest and apoptosis functions. It also binds TP73. Region II contains most of a central acidic region and a putative C4-type zinc finger. The RING finger domain which coordinates two molecules of zinc mediates the heterooligomerization with MDM2. Phosphorylated. Phosphorylation at Ser-367 promotes interaction with YWHAG and subsequent ubiquitination and degradation. Phosphorylation at Ser-341 also induces ubiquitination and degradation but to a lower extent (By similarity). Ubiquitinated and degraded by MDM2. Deubiquitination by USP2 on the other hand stabilizes the MDM4 protein (By similarity). Belongs to the MDM2/MDM4 family.