Characterizing the structure and function of a bacterial multi-kinase sensory complex

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Abstract Bacteria have an incredible capacity to sense and respond to intra- and extracellular fluctuations in the environment in order to maintain cellular homeostasis. In bacteria, environmental adaptation is commonly mediated by two-component systems (TCS) that consist of a sensor histidine kinase (HK) that phosphorylates a cognate response regulator (RR) in response to signal detection. Upon phosphorylation, the RR can bind to DNA and alter gene expression to facilitate environmental adaptation. Classical TCS have historically been thought to signal in a highly linear manner with minimal interaction or cross-regulation with other signaling pathways. A growing body of data from our group and others provide evidence that an unusual class of histidine kinases, known as HWE kinases, can form multi-protein signaling complexes, creating a new paradigm in bacterial signal transduction. These signaling systems can integrate information from numerous environmental inputs to coordinate an array of physiological responses. In Caulobacter crescentus, one such signaling complex, hereby referred to as the Alphaproteobacterial signalosome, has been identified to coordinately regulate cellular surface attachment, a critical initial step in biofilm formation. We have shown that the Alphaproteobacterial signalosome consists of a) the HWE kinase SkaH that functions as a molecular hub protein, b) the HWE kinase LovK, and c) the classical HK, SpdS. Individually, LovK and SpdS play critical roles in modulating the general stress response and stationary phase adaptation. Interestingly, sensory information from LovK and SpdS can be integrated through the signalosome to modulate cellular adhesion through the downstream transcription factors, RtrA and RtrB, and the hypothetical protein, RtrC. Preliminary data provides evidence that the signalosome is comprised of additional HWE and classical HK kinases, suggesting that the sensory complex can integrate a broader range of signals than previously suspected. The research proposed here takes a multidisciplinary approach to characterize the structure and function of the HWE signalosome. The first aim will use biochemical approaches and mass spectrometry to identify molecular partners of SkaH and dissect direct interactions within the signalosome. The second aim will complement the structural analysis of the signalosome by using biochemical approaches to analyze the signal flow through the signalosome components. Preliminary evidence suggests that the hypothetical protein, RtrC, is a cryptic transcription factor that functions as a critical output for the HWE signalosome. In the third aim, I will characterize the structure and function of RtrC with X-ray crystallography and fluorescent reporters. Additionally, I will use FRET-based biosensors and motility assays to examine the regulatory link between RtrC and c-di-GMP signaling. The HWE signalosome serves as a prime model system for examining how multi-kinase sensory systems detect and process complex environmental information in order to regulate physiological responses. Additionally, as HWE kinases are present in many bacterial pathogens, insights gained from this work will aid in the development of antibacterial therapies that target TCS.