The F-box proteins are the substrate recognition subunits of the SCF
(Skp1-Cul1-Rbx1-F- box protein) ubiquitin ligase complexes that control the
stability of numerous regulators in eukaryotic cells. Here we show that
dimerization of the F-box protein Fbx4 is essential for SCF(Fbx4) (the
superscript denotes the F-box protein) ubiquitination activity toward the
telomere regulator Pin2 (also known as TRF1). The crystal structure of Fbx4 in
complex with an adaptor protein Skp1 reveals an antiparallel dimer configuration
in which the linker domain of Fbx4 interacts with the C-terminal
substrate-binding domain of the other protomer, whereas the C-terminal domain of
the protein adopts a compact alpha/beta fold distinct from those of known F-box
proteins. Biochemical studies indicate that both the N-terminal domain and a
loop connecting the linker and C-terminal domain of Fbx4 are critical for the
dimerization and activation of the protein. Our findings provide a framework for
understanding the role of F-box dimerization in the SCF-mediated ubiquitination
reaction.
