Dr. Jianping Jin, Assistant Professor

ubiquitin signaling pathway & DNA Damage checkpoint control

Ubiquitin is a small signaling protein that covalently attaches to lysine residues of its substrates through its C-terminal glycine residue. This ubiquitylation process is mediated by three enzymes: a ubiquitin activating enzyme (E1), a ubiquitin conjugating enzyme (E2) and a ubiquitin ligase (E3). One of the major consequences of ubiquitylation is protein proteolysis through the 26S proteasome, which is the primary pathway that regulates the levels of many short-lived proteins in eukaryotes. The substrate specificity of ubiquitylation is governed by the E3s. Mutations in E3s or their cognate substrates result in substrate stabilization, often with deleterious consequences on cellular function. Dysregulation of the ubiquitin signaling pathway has been associated with numerous human diseases, such as cancer and neurodegenerative diseases.

1. System biology approaches to dissect new Uba6->Use1 ubiquitin signaling pathway

The first step of ubiquitylation is controlled by E1. It has been 25 years since the first E1 enzyme was identified, and it was believe that there was only one E1 for ubiquitin. Our recent discovery of a new E1 enzyme, Uba6, has reversed this 25-year-old concept. Uba6 is an essential gene for mouse development and only found in vertebrates and sea urchin. Uba6 transfers ubiquitin to its specific E2 enzyme, Use1, which is also limited in vertebrates and sea urchin. Serial biochemical and genetic experiments uncovered that Uba6 and Use1 constitute a unique activation and conjugation system for ubiquitin. Our lab takes a system biology approach to identify the downstream components of Uba6 and Use1, such as E3 ligases and substrates, and to characterize the biological function of this specific ubiquitin signaling pathway.

2. DNA damage checkpoint control by Cullin-RING ubiquitin ligases

Cullin-RING ubiquitin ligases (CRLs) constitute a super-family of multi-subunit E3 complexes. They consist of a ring finger protein, a cullin scaffold, an optional linker and a receptor subunit that usually has dozens of family members. CRLs control ubiquitylation of many important oncogenes and tumor suppressors, such as Cdc25A, c-Myc, c-Jun and p27. Emerging evidence indicates that CRLs play indispensable roles in the DNA damage checkpoint, cell division, DNA replication and repair, circadian rhythm, viral infection and chromatin remodeling etc. Mutations in components of CRLs have been found to be causative agents in certain cancers and other human diseases. Our laboratory focuses on the regulation of the DNA damage response through ubiquitylation by CRLs, particularly, by DCAF proteins that are the receptor subunits in CRL4 E3s.

 

 

Uba6 and Use1 are the newly identified ubiquitin E1 and E2 enzymes, respectively. Many important questions remain to be answered, such as their downstream components and their biological functions.

 

 

In healthy cells, DNA damage signals can trigger the degradation of many positive regulators of cell cycle and DNA replication in order to protect genome integrity. CRLs are the major ubiquitin ligases involving in those processes. For example, SCFβ-TRCP has been shown to ubiquitylate Cdc25A, an important protein phosphatase, while CRL4Cdt2 controls the proteolysis of Cdt1, a DNA replication licensing factor, upon DNA damage. However, the upstream signaling pathway leading to Cdt1 turnover is unclear.