In Schizosaccharomyces pombe, Cut8 is a nuclear envelope protein that physically interacts with and tethers 26S proteasome in the nucleus resulting in the nuclear accumulation of proteasome [1]. Cut8 comprises three functional domains. An N-terminal lysine-rich segment which binds to the proteasome when ubiquitinated, a central dimerisation domain and a C-terminal nine-helix, pdb:3q5w, bundle which shows structural similarity to 14-3-3 phosphoprotein-binding domains. The helical bundle is necessary for liposome and cholesterol binding [2]. Cut8 is a proteasome substrate and the N-terminal segment is polyubiquitinated and functions as a degron tag. Ubiquitination of the amino N-terminal segment is essential for the function of Cut8 [1]. Lysine residues in the N-terminal segment of Cut8 are required for physical interaction with proteasome [1]. In fission yeast the function of Cut8 has been demonstrated to be regulated by ubiquitin-conjugating Rhp6/Ubc2/Rad6 and ligating enzymes Ubr1 [1]. Cut8 homologues have been identified in Drosophila melanogaster, Anopheles gambiae and Dictyostelium discoideum [1].

1: Regulation of nuclear proteasome by Rhp6/Ubc2 through ubiquitination and destruction of the sensor and anchor Cut8. Takeda K, Yanagida M; Cell 2005;122:393-405. PMID:16096059

2: Implications for proteasome nuclear localization revealed by the structure of the nuclear proteasome tether protein Cut8. Takeda K, Tonthat NK, Glover T, Xu W, Koonin EV, Yanagida M, Schumacher MA; Proc Natl Acad Sci U S A. 2011;108:16950-16955. PMID:21976488