Ribosomal protein S3 is stabilized by sumoylation

Human ribosomal protein S3 (rpS3) acts as a DNA repair endonuclease. The multiple functions of this protein are regulated by post-translational modifications including phosphorylation and methylation. Using a yeast-two hybrid screen, we identified small ubiquitin-related modifier-1 (SUMO-1) as a new interacting partner of rpS3. rpS3 interacted with SUMO-1 via the N- and C-terminal regions. We also observed sumoylation of rpS3 in Escherichia coli and mammalian cell systems. Furthermore, we discovered that one of three lysine residues, Lys18, Lys214, or Lys230, was sumoylated in rpS3. Interestingly, sumoylated rpS3 was resistant to proteolytic activity, indicating that SUMO-1 increased the stability of the rpS3 protein. We concluded that rpS3 is covalently modified by SUMO-1 and this post-translational modification regulates rpS3 function by increasing rpS3 protein stability.     

Small ubiquitin-like modifier modification of arrestin-3 regulates receptor trafficking

Nonvisual arrestins are regulated by direct post-translational modifications, such as phosphorylation, ubiquitination, and nitrosylation. However, whether arrestins are regulated by other post-translational modifications remains unknown. Here we show that nonvisual arrestins are modified by small ubiquitin-like modifier 1 (SUMO-1) upon activation of β(2)-adrenergic receptor (β(2)AR). Lysine residues 295 and 400 in arrestin-3 fall within canonical SUMO consensus sites, and mutagenic analysis reveals that Lys-400 represents the main SUMOylation site. Depletion of the SUMO E2 modifying enzyme Ubc9 blocks arrestin-3 SUMOylation and attenuates β(2)AR internalization, suggesting that arrestin SUMOylation mediates G protein-coupled receptor endocytosis. Consistent with this, expression of a SUMO-deficient arrestin mutant failed to promote β(2)AR internalization as compared with wild-type arrestin-3. Our data reveal an unprecedented role for SUMOylation in mediating GPCR endocytosis and provide novel mechanistic insight into arrestin function and regulation.