In plants, there are a large number of F-box genes (697 in Arabidopsis thaliana compared to 11 in yeast - Kipreos and Pagano 2000; Gagne et al. 2002), suggesting that they may serve a variety of roles. F-box proteins have been implicated in hormone-regulated gene expression, flowering, and pho-tomorphogenesis (Gagne et al. 2002). At the biochemical level, they are best known for their role in ubiquitin-mediated protein degradation. Here, they act as substrate recognition factors for Skpl-Cullin-F-box (SCF) E3 ubiquitin ligases, which tag proteins for degradation. The F-box motif is typically near the N-terminus and binds to a Skpl-like protein. The C-terminal portion contains protein binding motifs (Gagne et al. 2002). In SCF, Skpl binds the F-box protein to a cullin protein. Other proteins, including RING (Really Interesting New Gene) domain proteins, also act as ubiquitin ligases to tag specific protein substrates (Willems et al. 2004). This led to the proposal of a model for SI based on specific protein degradation, which is discussed later.
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