F-box proteins: more than baits for the SCF?

Regulation of protein stability through the ubiquitin proteasome system is a key mechanism underlying numerous cellular processes. The ubiquitin protein ligases (or E3) are in charge of substrate specificity and therefore play a pivotal role in the pathway. Among the several different E3 enzyme families, the SCF (Skp1-Cullin-F box protein) is one of the largest and best characterized. F-box proteins, in addition to the loosely conserved F-box motif that binds Skp1, often carry typical protein interaction domains and are proposed to recruit the substrate to the SCF complex. Strikingly, genomes analysis revealed the presence of large numbers of F-box proteins topping to nearly 700 predicted in Arabidopsis thaliana. Recent evidences in various species suggest that some F-box proteins have functions not directly related to the SCF complex raising questions about the actual connection between the large F-box protein family and protein degradation, but also about their origins and evolution.