F-Box Proteins and Cancer

Controlled protein degradation is essential for the operation of a variety of cellular processes including cell division, growth, and differentiation. Identification of the relations between ubiquitin ligases and their substrates is key to understanding the molecular basis of cancer development and to the discovery of novel targets for cancer therapeutics. F-box proteins function as the substrate recognition subunits of S-phase kinase-associated protein 1 (SKP1)−Cullin1 (CUL1)−F-box protein (SCF) ubiquitin ligase complexes. Here, we summarize the roles of specific F-box proteins that have been shown to function as tumor promoters or suppressors. We also highlight proto-oncoproteins that are targeted for ubiquitylation by multiple F-box proteins, and discuss how these F-box proteins are deployed to regulate their cognate substrates in various situations

Comprehensive Identification of Substrates for F-box Proteins by Differential Proteomics Analysis

Although elucidation of enzyme–substrate relations is fundamental to the advancement of biology, universal approaches to the identification of substrates for a given enzyme have not been established. It is especially difficult to identify substrates for ubiquitin ligases, given that most such substrates are immediately ubiquitylated and degraded as a result of their association with the enzyme. We here describe the development of a new approach, DiPIUS (differential proteomics-based identification of ubiquitylation substrates), to the discovery of substrates for ubiquitin ligases. We applied DiPIUS to Fbxw7α, Skp2, and Fbxl5, three of the most well-characterized F-box proteins, and identified candidate substrates including previously known targets. DiPIUS is thus a powerful tool for unbiased and comprehensive screening for substrates of ubiquitin ligases

Comprehensive Identification of Substrates for F-box Proteins by Differential Proteomics Analysis

Although elucidation of enzyme–substrate relations is fundamental to the advancement of biology, universal approaches to the identification of substrates for a given enzyme have not been established. It is especially difficult to identify substrates for ubiquitin ligases, given that most such substrates are immediately ubiquitylated and degraded as a result of their association with the enzyme. We here describe the development of a new approach, DiPIUS (differential proteomics-based identification of ubiquitylation substrates), to the discovery of substrates for ubiquitin ligases. We applied DiPIUS to Fbxw7α, Skp2, and Fbxl5, three of the most well-characterized F-box proteins, and identified candidate substrates including previously known targets. DiPIUS is thus a powerful tool for unbiased and comprehensive screening for substrates of ubiquitin ligases

Comprehensive Identification of Substrates for F-box Proteins by Differential Proteomics Analysis

Although elucidation of enzyme–substrate relations is fundamental to the advancement of biology, universal approaches to the identification of substrates for a given enzyme have not been established. It is especially difficult to identify substrates for ubiquitin ligases, given that most such substrates are immediately ubiquitylated and degraded as a result of their association with the enzyme. We here describe the development of a new approach, DiPIUS (differential proteomics-based identification of ubiquitylation substrates), to the discovery of substrates for ubiquitin ligases. We applied DiPIUS to Fbxw7α, Skp2, and Fbxl5, three of the most well-characterized F-box proteins, and identified candidate substrates including previously known targets. DiPIUS is thus a powerful tool for unbiased and comprehensive screening for substrates of ubiquitin ligases

A stepwise and digital pattern of RSK phosphorylation determines the outcome of thymic selection

Summary: Developing CD4+CD8+ double-positive (DP) thymocytes with randomly generated T cell receptors (TCRs) undergo positive (maturation) or negative (apoptosis) selection on the basis of the strength of TCR stimulation. Selection fate is determined by engagement of TCR ligands with a subtle difference in affinity, but the molecular details of TCR signaling leading to the different selection outcomes have remained unclear. We performed phosphoproteome analysis of DP thymocytes and found that p90 ribosomal protein kinase (RSK) phosphorylation at Thr562 was induced specifically by high-affinity peptide ligands. Such phosphorylation of RSK triggered its translocation to the nucleus, where it phosphorylated the nuclear receptor Nur77 and thereby promoted its mitochondrial translocation for apoptosis induction. Inhibition of RSK activity protected DP thymocytes from antigen-induced cell death. We propose that RSK phosphorylation constitutes a mechanism by which DP thymocytes generate a stepwise and binary signal in response to exposure to TCR ligands with a graded affinity