Structural basis of dimerization-dependent ubiquitination by the SCF Fbx4 ubiquitin ligase

Abstract

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 SCFFbx4 (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 anti-parallel 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 α/β 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. © 2010 by The American Society for Biochemistry and Molecular Biology, Inc.