Architectural Basis of Leptin Transmembrane Signaling

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DESCRIPTION (provided by applicant): Leptin (L) and its receptor (L-R) are key players in the regulation of energy homeostasis and body weight. Complex formation between leptin and the extracellular portion of L-R results in the activation of Janus kinase 2 (JAK2) that is constitutively bound on the intracellular regions of the receptor. Leptin-instigated JAK2 signaling in hypothalamic nuclei reduces food intake and stimulates energy expenditure, while functional defects in either leptin or L-R result in morbid obesity, hyperglycemia, decreased insulin sensitivity, and hyperlipidemia. Despite the crucial impact of the leptin system on body weight and other physiological responses, little is known about the structure of the L/L-R complex and its association with JAK2. One of the reasons for this lack of insight is that both L-R and JAK2 have a relatively long and flexible multi-domain arrangement that has proved to be very challenging for both large-scale purification and implementation of X-ray crystallography. The present proposal aims to overcome these limitations in addressing the architectural prerequisites of L-R signaling by applying single- particle cryo-electron microscopy (cryo-EM) to characterize the holo-complex of full-length L/L-R and JAK2. Single-particle EM has emerged as a very powerful tool for the characterization of dynamic protein assemblies in relatively small concentrations and without the need for crystallization. We anticipate that the application of single-particle EM techniques on this system will reveal the architecture of the L/L-R assembly and JAK2 independently, and in complex. Given the underlying importance of this membrane-localized signaling complex in obesity, energy metabolism, and heart disease, the structural results obtained will be of very broad biomedical interest. Considering the current lack of structural information on any receptor/JAK complex, our studies will provide the general architectural framework for understanding how extracellular ligand binding on cytokine receptors results in intracellular JAK activation.