Structural Studies on Modular Sensory Proteins

The abilities to sense, respond, and adapt to environmental changes in various physical and chemical stimuli are essential for living organisms. Signaling proteins establish an intricate signaling network to ensure prompt and precise signal transduction from point A to B within a cell or from cell to cell. The primary function for a signaling protein is to relay information gathered from the internal or external environment to promote specific cellular processes, many of which are often linked to the cell fate. Hence, complex intracellular signaling networks are extensively studied for decades, which have yielded exciting discoveries of many essential signaling pathways that directly control cellular functions, and have provided a substantial leverage in therapeutic benefits. Despite these insights have showcased potential implications on developing therapeutic drugs to target specific signaling pathways, the current methods on pharmacological targeting of signaling pathway often lead to unexpected consequences such as increase in toxicity, acquired resistance, and reduced efficacy due to limited structural and mechanistic knowledge on signaling proteins. Bacterial signaling proteins have several characteristic features. First, signaling proteins often adopt a modular architecture, where each domain retains its core function. Hence, structures and functions of many isolated domains have been extensively studied. Second, these modular domains are typically seen in various arrangements assembled in a beans-on-the-stalk scaffold, where long linker helices join sensory domains in the N-terminal with output domains in the C-terminal. Hence, environmental signals captured in each sensory domain are subsequently transduced and integrated through the linker helices to control one output domain. Lastly, extensive structural studies of domains have revealed that signaling proteins are often found in a head-to-head dimeric scaffold. Interestingly, these homo dimers exhibit half-site reactivity. Such phenomenon is also observed in enzymes, which convey the idea that signaling proteins, as intricate molecular devices, are similarly designed as enzymes for specific purpose. In this work, phosphorylation assay, spectroscopy, site-directed mutagenesis, and dynamic crystallography are used to advance the current understanding of signal transduction mechanisms in modular sensory proteins.