Description
Post-translational modification with SUMO-1 regulates protein-protein interactions, localisation and stability. SUMOylation requires the E1 enzyme Aos1/Uba2 and the E2 enzyme Ubc9. A family of E3-like factors, PIAS proteins has been discovered, however, the nucleoporin RanBP2 also has SUMO-1 E3-like activity.
Modification of the histone deactylase HDAC4 depends upon the presence of an intact nuclear localisation signal and is catalysed by the nuclear pore complex (NPC) RanBP2 protein. This suggests that SUMOylation of HDAC4 takes place at the NPC and is coupled to nuclear import. This and other findings suggests that SUMOylation may be an important regulatory mechanism for the
control of transcriptional repression mediated by both class I and II HDACs.
Mdm2, a ubiquitin ligase that acts on the tumour suppressor p53, is regulated by SUMOylation. It has been shown that Mdm2 is SUMOylated during nuclear translocation by RanBP2 and then further SUMOylated once in the nucleus by PIASxbeta and PIAS1.
The RanGTPase activating protein RanGAP1 has essential functions in both nucleocytoplasmic transport and mitosis. In interphase, a significant fraction of vertebrate SUMO-1-modified RanGAP1 forms a stable complex with RanBP2 at nuclear pore complexes. RanGAP1 is subject to phosphorylation and it has been demonstrated that phosphorylated RanGAP1 remains associated with RanBP2 and Ubc9 in mitosis. Thus, mitotic phosphorylation may have functional consequences for the RanGTPase cycle and/or for RanBP2-dependent SUMOylation