Ubiquitin mechanism in health and pathogen-host interactions

Cornell University, Ithaca NY

Investigators

Abstract

Project Summary/Abstract Ubiquitination is a central mechanism that regulates a plethora of physiological processes in eukaryotes. Defects in the ubiquitination pathway are associated with many human diseases, including cancer and neurodegenerative diseases. Given the essential role of the ubiquitination pathway, it is not surprising that pathogens often exploit the host ubiquitin system to achieve their successful infection. My laboratory has a long-term interest in ubiquitin mechanisms in normal cell physiology and pathogen-host interactions. In collaboration with Dr. Scott Emr, we have discovered that several adaptor proteins mediating substrate ubiquitination by the Nedd4 family ubiquitin E3 ligases are themselves specifically di- ubiquitinated. We found that this modification is required for substrate ubiquitination, enhancing the interaction between the E3 ligases and their cognate adaptors, and promoting the intracellular recruitment of the E3s. However, the structural and biochemical basis of adaptor di-ubiquitination remains largely unknown. On the other hand, bacterial pathogens, such as Legionella pneumophila often exploit host ubiquitin to facilitate their intracellular proliferation. We have identified HECT-like ubiquitin ligases, secreted by the bacterium to hijack host ubiquitin. We also made significant contributions to the discovery and characterization of a novel type of ubiquitination, namely, the phosphoribosyl serine ubiquitin or PR- ubiquitination. Our laboratory also discovered specific DUBs that reverse PR-ubiquitination and an effector that catalyzes polyglutamylation on PR-ubiquitination ligases to inhibit the ligase activity in a Calmodulin- dependent manner. Built upon our previous findings, our future research under the MIRA will focus on two ubiquitin-related areas of research: 1) To elucidate the molecular mechanism of adaptor-mediated ubiquitination catalyzed by the Nedd4 subfamily of HECT E3 ligases; 2) To investigate the hijack of the host ubiquitin system by the intracellular bacterial pathogen, Legionella pneumophila. The characterization of specific di-ubiquitination of Nedd4 E3 adaptors will pave a new avenue to address key questions regarding how substrates are recognized and presented by adaptors and how the activity of Ne44d4 family E3 ligases is regulated. The study of ubiquitin hijacking in L. pneumophila infection is of also fundamental importance to our understanding of the versatile ubiquitin code. Since many Legionella effector proteins have eukaryotic origins evolutionarily. A fascinating future direction remaining to be addressed is whether there is a similar PR-ubiquitination system in eukaryotes. Taken together, our proposed in vivo and in vitro experiments using a combination of biochemical, cell biological, and structural biological approaches will enable us to make significant contributions to the understanding of novel molecular mechanisms of the ubiquitin system in normal cell physiology and in pathogen-host interactions.