Abstract (a) Introduction: The region of human chromosome 11p15.5 is linked with Beckwith-Wiedemann syndrome that is associated with susceptibility to Wilms’ tumor, rhabdomyosarcoma and hepatoblastoma. TSSC5 (Tumor Suppressing Subchromosomal Transferable fragment cDNA; also known as ORCTL2/ IMPT1/BWR1A/SLC22A1L) is located in the region. Mutations and/or reduced expression of TSSC5 have been found in certain tumors. Thus TSSC5 is a candidate for tumor suppressor, or the loss may serve as an additional factor in tumor progression. TSSC5 encodes an efflux transporter-like protein with ten transmembrane domains, and it may affect drug sensitivity, cellular metabolism and growth. However, the regulation of the TSSC5 protein is poorly understood. (b) Experiments: We identified a TSSC5-interacting protein by bacterial two hybrid assay, and further characterized the function of the protein in cultured cells and in enzyme assays in vitro. (c) Summary of novel findings: Here we present evidence indicating that RING105, a novel, conserved RING finger protein with a PA (Protease Associated) domain and a PEST sequence, is a ubiquitin ligase for TSSC5 that can function in concert with the ubiquitin-conjugating enzyme UbcH6. The polyubiquitin target site on TSSC5 was mapped to a region in the 6th hydrophilic loop. Ectopic expression of RING105 in HeLa cells caused an accumulation of cells in G1 that was not observed with expression of a mutant RING105 with impaired ligase activity. (d) Conclusions: Expression of RING105 induces a cell cycle delay at the G1/S transition indicating that RING105 may be a growth regulator. UbcH6-RING105 may define a novel ubiquitin-proteasome pathway that targets TSSC5 in mammalian cells. Because UbcH6 is a ubiquitin conjugating enzyme specialized to ERAD (Endoplasmic Reticulum-Associated Degradation) pathway, TSSC5 may be regulated through the ERAD. Manipulation of RING105 may be important for controlling cancers in which TSSC5 is misregulated.

[Proc Amer Assoc Cancer Res, Volume 47, 2006]