Portland State University. Department of Biology
Date of Publication
Master of Science (M.S.) in Biology
Ubiquitin, Protein binding
1 online resource (vi, 65 pages)
Modification of cellular proteins with molecules of ubiquitin is an important process that regulates the activity of cellular proteins. Cullin RING ligases (CRLs) are multi-subunit complexes that act in concert with E2 enzymes to attach molecules of ubiquitin to protein substrates. There are seven CRLs in mammalian cells (Cul1, Cul2, Cul3, Cul4A, Cul4B, Cul5, and Cul7) that are highly homologous in sequence and structure. CRLs possess a highly conserved C- terminal domain that interacts with E2 enzymes, and a more variable N- terminal domain which recruits substrates through distinct substrate adapter molecules. Despite the structural similarity, these CRLs recognize distinct substrates and carry out unique functions in cells.
In order to characterize the functional domains of cullins that are responsible for their unique activity, we generated cullin chimeras for expression and analysis in mammalian cells. These chimeras are Cul3 mutants in which the C- terminal domain or N- terminal domain of Cul3 has been replaced by that of Cul1 or Cul2, respectively. These chimeras were cloned into a mammalian expression vector for the purpose of experimentation in cultured cells.
The chimeric cullin constructs provided a valuable tool for investigating how different functional domains of CRLs contribute to their specific functions in cells. In this study, we first investigated if the chimeras that we engineered were able to interact with their respective substrate adapters. We performed co- immunoprecipitation experiments in which we tested the ability of wild type, chimeric, or mutant cullin proteins to bind to three different substrate adapter proteins. We found that the chimera possessing the C- terminus of Cul1 and the N- terminus of Cul3 retains the ability to interact with the BTB substrate adapters Ctb57 and KLHL3. We also found that the chimera that possesses the C- terminus of Cul3 and the N- terminus of Cul1 was unable to interact with BTB proteins. Lastly, we found that the Cul1 adapter Skp1 was able to bind to Cul1, but did not bind to Cul3 or either chimera. We concluded that the chimera possessing the N- terminus of Cul3 likely retains the functional binding abilities of Cul3 at the N- terminus and would therefore be useful for conducting experiments.
In this study, we also used the cullin chimeras to investigate the binding interactions between E2 enzymes and cullin RING ligases. We performed co- immunoprecipitation assays to examine the interactions between E2 enzymes and wild type, mutant or chimeric cullin proteins. We found that E2 enzyme UbE2E1 selectively binds to Cul3 and not to Cull. Notably, the BTB binding region at the N- terminus of Cul3 is required for binding to UbE2E1. Furthermore, we found that UbE2E1 also binds to Cul3 substrate adapter protein Ctb57. These experiments revealed a novel interaction between and E2 enzyme and the N- terminus of Cul3, as well as with a Cul3 substrate adapter protein. In conclusion, the chimeras generated in this study have provided valuable information regarding what regions of CRLs are important for interactions with other proteins, and will continue to be a useful tool for investigating CRL structure and function.
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Mitchell, Jennifer Anne, "Characterization of Functional Domains of Cul3, an E3 Ubiquitin Ligase, Using Chimeric Analysis" (2014). Dissertations and Theses. Paper 1970.