Hongjin Zheng - US grants

Project Summary Recent studies suggest that nitrate and nitrite molecules have profound beneficial effects to human health, though they were thought to be cancerous since 1970s. To maximize their pharmaceutical potential, we need to first understand how nitrate and nitrite circulate in humans. Such circulation depends on two critical events: active accumulation of nitrate mediated by sialin transporters in salivary glands, as well as nitrate uptake and nitrite secretion by commensal bacteria in the mouth. Thus, nitrate/nitrite molecules have to cross different cellular membranes multiple times, before being used by humans. These translocation processes are mediated by a group of membrane proteins called nitrate transporters. To understand how these transporters function, we solved high-resolution crystal structures of NarK from E. coli, and further demonstrated that, surprisingly, NarK is a nitrate/nitrite exchanger. Despite the progress we made, the detailed molecular mechanisms of nitrate/nitrite translocation are still largely unknown. In this proposal, we aim to fill the knowledge gap by: 1) understand substrate selectivity and conformational flexibility using directed-mutagenesis of NarK, so to better understand the function of NarK at the molecular level; 2) obtain high-resolution structures of NarK in previously unobserved conformation, so we can reconstruct the complete transport cycle of NarK; 3) explore the structure-function relationship of human nitrate transporter sialin, so we will understand the similarities and differences among nitrate transporters from diverse species. Overall, upon completion of the proposal, we expect to expand our general understanding of the nitrate/nitrite transport, and shed light on the crucial roles that nitrate transporters (both eukaryotic and prokaryotic) play in nitrate/nitrite circulation. The knowledge gained here will facilitate potential drug development related to bacterial nitrate transporters and human sialin.

Project Summary Recent studies suggest that nitrate and nitrite molecules have profound beneficial effects to human health, though they were thought to be cancerous since 1970s. To maximize their pharmaceutical potential, we need to first understand how nitrate and nitrite circulate in humans. Such circulation depends on two critical events: active accumulation of nitrate mediated by sialin transporters in salivary glands, as well as nitrate uptake and nitrite secretion by commensal bacteria in the mouth. Thus, nitrate/nitrite molecules have to cross different cellular membranes multiple times, before being used by humans. These translocation processes are mediated by a group of membrane proteins called nitrate transporters. To understand how these transporters function, we solved high-resolution crystal structures of NarK from E. coli, and further demonstrated that, surprisingly, NarK is a nitrate/nitrite exchanger. Despite the progress we made, the detailed molecular mechanisms of nitrate/nitrite translocation are still largely unknown. In this proposal, we aim to fill the knowledge gap by: 1) understand substrate selectivity and conformational flexibility using directed-mutagenesis of NarK, so to better understand the function of NarK at the molecular level; 2) obtain high-resolution structures of NarK in previously unobserved conformation, so we can reconstruct the complete transport cycle of NarK; 3) explore the structure-function relationship of human nitrate transporter sialin, so we will understand the similarities and differences among nitrate transporters from diverse species. Overall, upon completion of the proposal, we expect to expand our general understanding of the nitrate/nitrite transport, and shed light on the crucial roles that nitrate transporters (both eukaryotic and prokaryotic) play in nitrate/nitrite circulation. The knowledge gained here will facilitate potential drug development related to bacterial nitrate transporters and human sialin.

Compelling evidence has suggested that mitochondrial dysfunction is an early event in Alzheimer?s Disease (AD) pathophysiology. This mitochondrial dysfunction is closely related to the elevated level of intracellular A?, which appears to be accumulated within mitochondria in the brains of both Alzheimer?s patients and mouse models. How mitochondria actively accumulate A? is not clear, and thus represents a large gap of knowledge in the field. We aim to understand the very first step of this accumulation process by studying the detailed molecular mechanism of substrate/receptor interaction. We have identified that A? is recognized by a non-canonical receptor, Tom22, within the mitochondrial protein import machinery. In this proposal, we expect to perform two related but independent specific aims to reach that goal. 1). We will characterize the interaction between A? and Tom22 receptor in details, by biophysical, biochemical and cell biology tools; 2). We expect to determine three dimensional atomic structures of Tom22 and Tom22/A? complex. The functional and structural information gained here are expected to reveal detailed molecular mechanism underlining the detrimental process of mitochondrial uptake of A? peptides, and thus provide novel models to screen molecules that are capable of disrupting the specific Tom22/A? interaction. Thus, the outcome of this proposal is expected to have important positive impact in treating mitochondrial dysfunction caused by A? in AD.

Recent studies suggest that nitrate and nitrite molecules have profound beneficial effects to human health, though they were thought to be cancerous since 1970s. To maximize their pharmaceutical potential, we need to first understand how nitrate and nitrite circulate in humans. Such circulation depends on two critical events: active accumulation of nitrate mediated by sialin transporters in salivary glands, as well as nitrate uptake and nitrite secretion by commensal bacteria in the mouth. Thus, nitrate/nitrite molecules have to cross different cellular membranes multiple times, before being used by humans. These translocation processes are mediated by a group of membrane proteins called nitrate transporters. To understand how these transporters function, we solved high-resolution crystal structures of NarK from E. coli, and further demonstrated that, surprisingly, NarK is a nitrate/nitrite exchanger. Despite the progress we made, the detailed molecular mechanisms of nitrate/nitrite translocation are still largely unknown. In this proposal, we aim to fill the knowledge gap by: 1) understand substrate selectivity and conformational flexibility using directed-mutagenesis of NarK, so to better understand the function of NarK at the molecular level; 2) obtain high-resolution structures of NarK in previously unobserved conformation, so we can reconstruct the complete transport cycle of NarK; 3) explore the structure-function relationship of human nitrate transporter sialin, so we will understand the similarities and differences among nitrate transporters from diverse species. Overall, upon completion of the proposal, we expect to expand our general understanding of the nitrate/nitrite transport and shed light on the crucial roles that nitrate transporters (both eukaryotic and prokaryotic) play in nitrate/nitrite circulation. The knowledge gained here will facilitate potential drug development related to bacterial nitrate transporters and human sialin.