Human Regulatory Protein Ki-1/57 Is a Target of SUMOylation and Affects PML Nuclear Body Formation

Ki-1/57 is a nuclear and cytoplasmic regulatory protein first identified in malignant cells from Hodgkin’s lymphoma. It is involved in gene expression regulation on both transcriptional and mRNA metabolism levels. Ki-1/57 belongs to the family of intrinsically unstructured proteins and undergoes phosphorylation by PKC and methylation by PRMT1. Previous characterization of its protein interaction profile by yeast two-hybrid screening showed that Ki-1/57 interacts with proteins of the SUMOylation machinery, the SUMO E2 conjugating enzyme UBC9 and the SUMO E3 ligase PIAS3, which suggested that Ki-1/57 could be involved with this process. Here we identified seven potential SUMO target sites (lysine residues) on Ki-1/57 sequence and observed that Ki-1/57 is modified by SUMO proteins in vitro and in vivo. We showed that SUMOylation of Ki-1/57 occurred on lysines 213, 276, and 336. In transfected cells expressing FLAG-Ki-1/57 wild-type, its paralog FLAG-CGI-55 wild-type, or their non-SUMOylated triple mutants, t...