Characterization of the Substrate Specificity and the Cellular Function of the SCFPof3 Ubiquitin Ligase Complex

Ubiquitination and subsequent degradation of proteins regulates a wide variety of cellular processes. Many key activators and inhibitors of cell division are targeted for degradation by a recently described family of E3 ubiquitin protein ligases termed Skp1-Cdc53-F-box protein (SCF) complexes. In this enzyme complex the F-box proteins, as a variable part, act as receptors to recruit substrates to the complex conferring the specificity of the ubiquitin transfer. In yeast and mammals F-box proteins were shown to recruit cyclins, cell cycle inhibitors or transcription factors thereby regulating steps of cell cycle progression and signal transduction. Therefore, defects in SCF complexes have been implicated in tumorigenesis. Recently a novel F-box protein, Pof3, of Schizosaccharomyces pombe has been identified. In the absence of Pof3, cells exhibit a number of phenotypes including G2 cell cycle delay, hypersensitivity to UV irradiation, a high rate of chromosome loss as well as telomere shortening and alleviation of the transcriptional silencing at the telomere. Although the substrates of Pof3 are not known, these phenotypes point to a central role of Pof3 in protection of genome integrity. This project aims to identify the substrates that are bound and recruited by Pof3. Characterization of the interaction partners and their cellular function will enable us to gain further insight into the processes that involve the SCFPof3 ligase.