A combination of the F-box motif and kelch repeats defines a large Arabidopsis family of F-box proteins

In the sequences released by the Arabidopsis Genome Initiative (AGI), we ha ve discovered a new large gene family (48 genes as of July 2000). A detaile d computational and biochemical analysis of the predicted gene products rev eals a novel family of plant F-box proteins, where the amino (N)-terminal F -box motif is followed by four kelch repeats and a characteristic carboxy-t erminal domain. F-box proteins are an expanding family of eukaryotic protei ns, which have been shown in some cases to be critical for the controlled d egradation of cellular regulatory proteins via the ubiquitin pathway. The F -box motif of the At5g48990 gene product, a member of the family, was shown to be functionally active by its ability to mediate the in vitro interacti on between At5g48990 and ASK1 proteins. F-box proteins specifically recruit the targets to be ubiquitinated, mainly through protein-protein interactio n modules such as WD-40 domains or leucine-rich repeats (LRRs). The kelch r epeats of the family described here form a potential protein-protein intera ction domain, as molecular modelling of the kelch repeats according to the galactose oxidase crystal structure (the only solved structure containing k elch repeats) predicts a beta -propeller. The identification of this family of F-box proteins greatly expands the field of plant F-box proteins and su ggests that controlled degradation of cellular proteins via the ubiquitin p athway could play a critical role in multiple plant cellular processes.