Ubiquitin Specific Protease 11 (USP11): structure and enzymology

点击翻译按钮获取中文摘要

The units that make up an organism, the cells, rely on the presence of a large number of proteins that are required at different times. In order to quickly respond to environmental changes or to dispose of damaged proteins, cells harbour an effective system for protein destruction that is tightly regulated as any mistakes can have serious effects for the cell. Protein destruction consists of a series of events. At first a small protein termed ubiquitin is attached to a protein molecule that serves as a signal to direct these molecules into a certain pathway. The formation of a particular type of chain of several ubiquitin molecules on a protein results in them being recognised by a large molecular machine, the proteasome that ultimately digests the protein into small pieces. The attachment of only one ubiquitin moiety or other types of ubiquitin chain to a protein can also lead to other outcomes unrelated to protein destruction. Because the correct timing is essential, there are mechanisms to remove or alter these signals. Check point molecules called deubiquitinating enzymes can remove ubiquitin molecules and consequently alter the fate of a target protein. The most abundant class of these deubiquitinating enzymes in humans are the ubiquitin specific proteases (USPs). They regulate ubiquitin-dependent metabolic pathways by cleaving ubiquitin-protein bonds. Each USP specifically recognizes a limited number of ubiquitinated proteins that they can salvage from destruction or channel into different pathways within cells. The structure of these molecules is complicated in that they need to be able to provide binding sites for the protein, in order to be specific, as well as the attached ubiquitin chain and be highly selective in their action. Removing the ubiquitin signal from the wrong protein could have disastrous effects for the cell. This research aims at gaining insights into how one ubiquitin specific protease, USP11, achieves this specificity, whether it is able to distinguish between different ubiquitin chains and how its molecular make-up influences its function. To this end we will determine structures of discrete parts of USP11 and identify binding surfaces for the protein and the ubiquitin chain. As USP11 also interacts with other proteins we will look for additional binding sites. A second attempt to gain a better understanding of these important regulatory proteins will consist of looking at the shape and flexibility of the whole molecule in solution. Ultimately we will establish which parts of USP11 are involved in the recognition of the viral protein HPV-16E7 that once infection has occurred exploits the function of USP11 in order to extend its own life-span. Together, this will provide unique novel insights into how these proteases work, which will help us to better understand this check-point system in regulating normal protein destruction pathways, signalling events and in viral infection.