New interacting partners of the F-box protein Ufo1 of yeast

The yeast F-box protein Ufo1 recruits proteins for ubiquitylation by the SCF ubiquitin ligase complex preparing them for proteasomal degradation. Ufo1 has a role in maintenance of genome stability; its substrates include Ho endonuclease and Rad30 polymerase of error-prone DNA repair. Ufo1 is an unusual F-box protein, as it has three ubiquitin interacting motifs (UIMs). Deletion of the genomic UIMs is lethal; ectopic expression of UF01 Delta U1Ms extends protein half-life and arrests the cell cycle. A whole-genome study employing a TAP tag fused to the C-terminal UlMs did not identify Ufo1-interacting proteins. Here we therefore used stabilized N-terminally tagged Ufo1 Delta UIM as a strategy to identify Ufo1-interacting proteins by mass spectroscopy. We identified proteins that function in transcription, and an indirect interaction with Hsp70 molecular chaperones via the Skp1 adaptor; we also show that Ufo1 interacts with the 19S regulatory particle of the proteasome. Thus, our data augment the current network of known Ufo1 interacting proteins. We show directly that the UlMs are crucial for Ufo1 ubiquitylation in vivo, indicating that they facilitate turnover of SCFUf01 complexes. This allows recycling of the core subunits of the SCIF complex and cell cycle progression. Copyright (C) 2008 John Wiley & Sons, Ltd.