The alpha helix of the intermediate region in hGBP-1 acts as a coupler for enhanced GMP formation

Human guanylate binding proteins: nanomachines orchestrating host defense

Disease-causing microorganisms not only breach anatomical barriers and invade tissues but also frequently enter host cells, nutrient-enriched environments amenable to support parasitic microbial growth. Protection from many infectious diseases is therefore reliant on the ability of individual host cells to combat intracellular infections through the execution of cell-autonomous defense programs. Central players in human cell-autonomous immunity are members of the family of dynamin-related guanylate binding proteins (GBPs). The importance of these interferon-inducible GTPases in host defense to viral, bacterial, and protozoan pathogens has been established for some time; only recently, cell biological and biochemical studies that largely focused on the prenylated paralogs GBP1, GBP2, and GBP5 have provided us with robust molecular frameworks for GBP-mediated immunity. Specifically, the recent characterization of GBP1 as a bona fide pattern recognition receptor for bacterial lipopolysaccharide (LPS) disrupting the integrity of bacterial outer membranes through LPS aggregation, the discovery of a link between hydrolysis-induced GMP production by GBP1 and inflammasome activation, and the classification of GBP2 and GBP5 as inhibitors of viral envelope glycoprotein processing via suppression of the host endoprotease furin have paved the way for a vastly improved conceptual understanding of the molecular mechanisms by which GBP nanomachines execute cell-autonomous immunity. The herein discussed models incorporate our current knowledge of the antimicrobial, proinflammatory, and biochemical properties of human GBPs and thereby provide testable hypotheses that will guide future studies into the intricacies of GBP-controlled host defense and their role in human disease.

Stimulation of GMP formation in hGBP1 is mediated by W79 and its effect on the antiviral activity

Interferon-inducible large GTPases are critical for innate immunity. The distinctive feature of a large GTPase, human guanylate binding protein-1 (hGBP1), is the sequential hydrolysis of GTP into GMP via GDP. Despite several structural and biochemical studies, the underlying mechanism of assembly-stimulated GMP formation by hGBP1 and its role in immunity are not fully clarified. Using a series of biochemical, biophysical, and in silico experiments, we studied four tryptophan residues, located near switch I-II (in and around the active site) to understand the conformational changes near these regions and also to investigate their effect on enhanced GMP formation. The W79A mutation showed significantly reduced GMP formation, whereas the W81A and W180A substitutions exhibited only a marginal defect. The W114A mutation showed a long-range effect of further enhanced GMP formation, which was mediated through W79. We also observed that after first phosphate cleavage, the W79-containing region undergoes a conformational change, which is essential for stimulated GMP formation. We suggest that this conformational change helps to reposition the active site for the next cleavage step, which occurs through a stable contact between the indole moiety of W79 and the main chain carbonyl of K76. We also showed that stimulated GMP formation is crucial for antiviral activity against hepatitis C. Thus, the present study not only provides new insight for the stimulation of GMP formation in hGBP1, but also highlights the importance of the enhanced second phosphate cleavage product in the antiviral activity.