A yeast 2-hybrid screen to identify and characterize interaction partners of the cancer associated protein retinoblastoma binding protein 6

Abstract

Retinoblastoma binding protein 6 (RBBP6) is a 250 kDa protein that is implicated in mRNA processing and ubiquitination functions and has been shown to be highly up-regulated in a number of cancers. In humans and mice,RBBP6 interacts with both tumour suppressors p53 and pRb, suggesting that it is involved in regulation of transcription, induction of apoptosis and cell cycle control. Knock-out of an RBBP6 homologue PACT resulted in p53 dependent cell cycle arrest and apoptosis. Although the biological functions of RBBP6 remain largely unclear, it is possible that its functions are mediated through interaction with other cellular proteins. Since it is possible to unveil novel functions of a target protein through identifying its interacting protein partners,this study aims to further characterize the functions of RBBP6 through identifying novel protein interacting partners using a yeast 2-hybrid screen.In order to identify interaction partners of RBBP6, two well characterized domains of RBBP6, the N-terminal ubiquitin-like DWNN domain and RING finger domain, were used as baits in a yeast 2-hybrid screen of a human testis cDNA library. Putative interactors were verified using in vitro and in vivo immunoprecipitation assays. The RING finger domain was shown to interact with transcriptional factors Y-Box binding protein 1 (YB-1) and zinc finger and BTB containing protein 38 (zBTB38), resulting in their ubiquitination. In the case of YB-1 ubiquitination was correlated with a decrease in the intra-cellular levels of YB-1, suggesting that ubiquitination leads to degradation in the proteosome. The DWNN domain was shown to interact with a splicing associated small nuclear ribonucleoprotein polypeptide G (snRPG) and heat shock protein 70 (Hsp70).The results of this work suggest that, at least in the case of YB-1 and zBTB38,RBBP6 plays a role in the regulation of gene expression by ubiquitination of transcription factors, causing them to be degraded in the proteosome. The study provides further evidence of RBBP6’s involvement in mRNA splicing through its interaction with snRPG. The interaction with Hsp70 suggests a possible role in protein quality control similar to that played by other E3 ligases such as Parkin and CHIP.