Abstract Tumor susceptibility gene TSG101 plays an important role in cellular functions including intracellular protein sorting, vesicular trafficking, and transcription regulation. Our previous results from yeast two-hybrid screening show that TSG101 interacts with a novel transcriptional repressor protein, KLIP1. In this study, we demonstrated in vivo interaction between TSG101 and KLIP1 in nucleus of 293 cells using co-immunoprecipitation assay and confocal imaging. In addition, we found KLIP1 could be modified in a modality of either poly- and mono-ubiquitination when exogenously expressed in 293 cells in conjunction with either wild type His-tagged ubiquitin or a mutant His-tagged ubiquitin (K0-Ub) which has no capability of forming polyubiquitin chain. Furthermore, we found that TSG101 could increase 60 kDa-KLIP1, but decrease 71 kDa-KLIP1 levels of monoubiquitinated KLIP1 protein species in a dose dependent manner. These results indicate that TSG101 might regulate KLIP1 protein function through affecting its monoubiquitin modification status. Further investigation using wildtype pHA-KLIP1 and mutant pHA-KLIP1-M6 containing mutation in its 6 lysine residues for possible ubiquitin modification revealed that wildtype HA-KLIP1, but not HA-KLIP1-M6, could inhibit transcription activity of thymidine kinase (TK) promoter. In conclusion, our results support that TSG101 interacts and acts as a transcriptional co-repressor of KLIP1 by keeping it in 60 kDa-monoubiquitinated status in the nucleus, where KLIP1 functions as a transcription repressor for TK promoter. Further experiment using mutant HA-KLIP1 expression plasmid containing single mutation in the 6 lysine sites should reveal the exact location of ubiquitin-modified lysine site for monoubiquitinated species of KLIP1 protein.

Tumor Susceptibility Gene, TSG101, has been identified as a tumor susceptibility gene with multiple functions. TSG101 encodes a 46 kDa protein composed of 390 amino acids. As previous studies reported, TSG101 participates in cell-cycle control, membrane proteins¡¦ trafficking, and transcriptional regulation. To identify the proteins that mediated function involved TSG101, we perform yeast two-hybrid cDNA library screening to search for TSG101-interacting proteins. A construct pAS2-1-TSG101 was used as a bait to screen a human testis cDNA library. This screening selected 68 TSG101 interacting clones, including 17 known proteins. These proteins were functionally classified as participating in cell-cycle alteration, protein sorting, transcriptional regulation, modification, signal transduction and other functions. Our results provide the evidences which not only confirm the results of previous studies, but also provide further information related to TSG101 biological functions worth intensive study. Among these clones, we choose KLIP1 gene, which encodes a transcription factor, for further study to elucidate the functional role of TSG101 in nucleus. In vitro GST pull-down assay and in vivo co-immunoprecipitation assay were performed using GST-KLIP1 and HA-tagged KLIP1, respectively, have demonstrated that TSG101 and KLIP1 indeed interact with each other within mammalian cells. Detailed biological function mediated through this TSG101 and KLIP1 interaction awaits further investigation.

Nuclear protein KLIP1 cooperates with myeloid leukemia factor 1 (MLF1) to inhibit the programmed cell death resulting in tumor formation. It also inhibits the activity of thymidine kinase promoter of Kaposi¡¦s sarcoma-associated Herpes Virus. KLIP1 functions as a centromere protein, hence acquires its name as CENP-U or CENP-50, to regulate the separation of sister-chromatids during mitosis. These results indicate that KLIP1 plays important roles in regulation of transcription and cell cycle. In this study, six potential SUMO modification sites, K33, K63, K126, K127, K185 and K210, were identified bioinformatically using SUMOplot. Many reports address that SUMO modification alters the transcriptional activity, protein-protein interaction, the subcellular localization and stability of its target protein. Recent data suggest that SUMO is required for centromere binding protein to mediate proper mitotic spindle attachment to the kinetochore, and previous research suggest that there has a SUMO-interaction motif (SIM) in KLIP1 protein sequence. To reveal the interaction between KLIP1 and SUMO-1, and study its effects on KLIP1 function, we co-express GFP-KLIP1 and His-tagged SUMO-1 in HEK 293 cells. After affinity purification of SUMOylated proteins from transfected cells using nickel conjugated beads and subsequent western blotted with anti-GFP. The results indicated the interaction between KLIP1 and SUMO-1 in co-transfected cells. Our confocal microscopy imaging also found colocalization of GFP-KLIP1 with RFP-SUMO-1 nuclear foci. In addition, we failed to detect the interaction between SUMO-1 and mutant KLIP1-M6 ,whose six potential SUMO modified lysine residues were mutated to arginine. Furthermore, we found a distinct nuclear localization of GFP-KLIP1-M6 as compared to the image of wildtype GFP-KLIP1, which show a significant higher frequency of colocalization with RFP-SUMO-1 foci. Taken together, our data suggest the interaction between KLIP1 and SUMO-1 may be related to these six potential lysine residues, which upon mutation blocks its colocalization with SUMO-1 in nuclear foci. The biological significance of their interaction are awaits for further investigation.