In all human cells, damaged or unnecessary proteins are destroyed through a complex process that involves their being tagged with chains of a small protein called ubiquitin. The ubiquitin-tagged proteins are then directed to a sophisticated cellular structure known as the proteasome, which degrades the proteins.
The addition of ubiquitin to targeted proteins, a process called ubiquitylation, takes place in a multistep process in which several types of proteins, or enzymes, function in a sequential, bucket brigade-like manner. First, ubiquitin is activated by an enzyme known as E1. The activated ubiquitin is then transferred to E2, another enzyme. The E2 binds in turn to another protein known as E3, or ubiquitin protein ligase, which is critical for the transfer of ubiquitin to the targeted protein. This process occurs in a highly regulated manner that allows the recognition and targeting of specific proteins. To achieve the necessary specificity, human cells have about 40 different types of E2 enzymes and more than 500 E3 proteins.
A majority of E3s have an internal structural component, or domain, known as a RING finger, which binds weakly to E2s and allows ubiquitylation to proceed. An earlier study by one of the lead authors of the current report, Allan M. Weissman, M.D., of NCI's Center for Cancer Research (CCR), discovered that a RING finger E3, known as gp78, has a unique region called G2BR that strongly binds to its E2. Weissman and other NCI scientists previously showed that higher levels of gp78 promote the spread of cancer by tagging a protein for degradation that suppresses metastasis and that the ubiquitin ligase activity of gp78 was required for this degradation. Other targets of gp78 include proteins that are involved in cystic fibrosis and in the regulation of lipid metabolism.
In this new work, a team of CCR researchers, led by R. Andrew Byrd, Ph.D., Xinhua Ji, Ph.D., and Weissman, used advanced structural techniques to study the structure of gp78 and its associated E2 enzyme to gain insight into how the complex functions in cells. The researchers determined the structural basis for the interaction between gp78 and its E2 and uncovered a previously unknown mechanism by which ubiquitylation can be regulated. They found that the gp78 G2BR binds its E2 in an area that is distinct from the sites where the gp78 RING finger domain binds to the E2. This binding causes subtle changes in the shape of the E2 that allow the gp78 RING finger domain and the E2 to join together 50 times more tightly than they otherwise would. Further research showed that this increased binding strength enhances ubiquitylation of target proteins by gp78.
This discovery may allow researchers to consider possible approaches to blocking the function of gp78 in cancer cells, leading to new types of treatment for cancer and other diseases. "Our study provides a previously unappreciated mechanism by which ubiquitylation can be regulated," said Weissman. "It is likely that other pairs of E2s and E3s interact through domains, which have yet to be characterized, that are similar to the gp78 G2BR and its corresponding binding site on its E2. This introduces the possibility of entirely new therapeutic avenues in cancer and other diseases."
This team is currently working to further define the interactions of E2s and RING finger domains. They also are collaborating with other NCI scientists to design and construct potential inhibitors of gp78, based on their discovery, for testing in animal models.