The Highs and Lows of FBXW7: New Insights into Substrate Affinity in Disease and Development

Abstract

FBXW7 is a critical regulator of cell cycle, cell signaling, and development. A highly conserved F-box protein and component of the SKP1-Cullin-F-box (SCF) complex, FBXW7 functions as a recognition subunit within a Cullin-RING E3 ubiquitin ligase responsible for ubiquitinating substrate proteins and targeting them for proteasome-mediated degradation. In human cells, FBXW7 promotes degradation of a large number of substrate proteins, including many that impact disease, such as NOTCH1, Cyclin E, MYC, and BRAF. A central focus for investigation has been to understand the molecular mechanisms that allow the exquisite substrate specificity exhibited by FBXW7. Recent work has produced a clearer understanding of how FBXW7 physically interacts with both high-affinity and low-affinity substrates. We review new findings that provide insights into the consequences of "hotspot" missense mutations of FBXW7 that are found in human cancers. Finally, we discuss how the FBXW7-substrate interaction, and the kinases responsible for substrate phosphorylation, contribute to patterned protein degradation in C. elegans development.

Keywords: E3 ubiquitin ligase; FBXW7/FBW7/Cdc4/SEL-10; phospho-degron; ubiquitin–proteasome system.

Figure 1

FBXW7 interaction with high and low-affinity substrates. (A) Domains of the FBXW7 protein include three different N-termini produced from alternative isoforms and common regions that include the dimerization domain, F-box domain, and WD40 repeat domain. The SCF-FBXW7 E3 ubiquitin ligase is a complex comprising FBXW7, SKP1, CUL1, and RBX1. This complex is capable of recruiting an E2 ubiquitin ligase through RBX1 and CUL1. FBXW7 acts as the substrate recognition component, binding a substrate CPD. (B–D) Examples of FBXW7 binding with high and low affinity. At left, the FBXW7 WD40 repeat domain surface is semi-transparent, with sidechains involved in CPD binding shown (blue). The CPD peptide (tan) sidechains are shown for P-2, P-1, P0, and P+4 positions. Red is used to indicate negative charges. At right, FBXW7 binding residues (blue) and CPD (tan) are shown without the surface rendering. (B) The high-affinity CPD at Cyclin E T380 contains three components: P-2, P-1 hydrophobic leucines, P0 pThr, and P+4 pSer. Binding is coordinated by FBXW7 residues Y519, R505, R479, R465, T463, S462, and R441. (C) The low-affinity CPD at MYC T244 has negatively charged glutamates rather than hydrophobic residues at P-2 and P-1. (D) The low affinity CPD at Cyclin T62 does not contain a phosphorylated P+4 residue. In this structure, positions after P+1 were disordered. Illustrations were generated from PDB accessions 2OVQ, 7TY1, 2OVR [10,14].

Figure 2

Patterns of FBXW7 degradation for high- and low-affinity substrates. (A) The C. elegans Raf ortholog LIN-45 contains one known CPD that conforms to t he high-affinity consensus. LIN-45 protein and conserved domains (green), including the Ras-binding domain (RBD), the cysteine-rich domain (CRD), and the kinase domain (KD). CPD sequence is shown below. (B) At the C. elegans L3 larval stage, a population of cells termed VPCs express LIN-45 protein (green). Although initially expressed in all VPCs, LIN-45 is degraded in one denoted P6.p. Degradation of LIN-45 requires the C. elegans FBXW7 ortholog SEL-10 and the kinases ERK, CDK2, and GSK3. At the L3 stage, elimination of LIN-45 closely matches the pattern of ERK activation, with high ERK activation and LIN-45 degradation both occurring specifically in P6.p ERK activation is summarized as low or high for each cell (magenta). (C) The C. elegans RNA-binding TRIM71 ortholog LIN-41 contains multiple, non-consensus degrons. LIN-41 protein and conserved domains (orange), including the NHL repeats (NHL). Degron A, B1, and B2 are non-overlapping regions required for LIN-41 degradation and enriched in Ser/Thr-Pro sequences. (D) During C. elegans oogenesis, maturing oocytes in late prophase express LIN-41 protein (orange). LIN-41 is seen in oocytes just after fertilization but is rapidly eliminated as they complete the first division of meiosis. LIN-41 degradation requires SEL-10 and the kinase CDK1. Activation of CDK1 is presumed to increase as oocytes progress from late prophase to the meiosis I division, as summarized for late prophase, fertilization, and meiosis I (blue). In contrast, LIN-41 degradation occurs precisely at the division of meiosis I. Illustrations summarize data from [43,47].