title: Graded Structural Polymorphism in a Bacterial Thermosensor Protein.
abstract: Thermosensing is critical for the expression of virulence genes in pathogenic bacteria that infect warm-blooded hosts. Proteins of the Hha-family, conserved among enterobacteriaceae, have been implicated in dynamically regulating the expression of a large number of genes upon temperature shifts. However, there is little mechanistic evidence at the molecular level as to how changes in temperature are transduced into structural changes and hence the functional outcome. In this study, we delineate the conformational behavior of Cnu, a putative molecular thermosensor, employing diverse spectroscopic, calorimetric and hydrodynamic measurements. We find that Cnu displays probe-dependent unfolding in equilibrium, graded increase in structural fluctuations and temperature-dependent swelling of the dimensions of its native ensemble within the physiological range of temperatures, features that are indicative of a highly malleable native ensemble. Site-specific fluorescence and NMR experiments in combination with multiple computational approaches-statistical mechanical model, coarse-grained and all-atom MD simulations-reveal that the fourth helix of Cnu acts as a unique thermosensing module displaying varying degrees of order and orientation in response to temperature modulations while undergoing a continuous unfolding transition. Our combined experimental-computational study unravels the folding-functional landscape of a natural thermosensor protein, the molecular origins of its unfolding complexity, highlights the role of functional constraints in determining folding-mechanistic behaviors, and the design principles orchestrating the signal transduction roles of the Hha protein family.
authors.affiliation: Department of Biotechnology, Bhupat & Jyoti Mehta School of Biosciences, Indian Institute of Technology Madras (IITM) , Chennai 600036, India.
authors.forename: Abhishek
authors.initials: A
authors.lastname: Narayan
authors.affiliation: National Biotechnology Center , Consejo Superior de Investigaciones Científicas, Darwin 3, Campus de Cantoblanco, 28049 Madrid, Spain.
authors.forename: Luis A
authors.initials: LA
authors.lastname: Campos
authors.affiliation: National Centre for Biological Sciences (NCBS) , Tata Institute of Fundamental Research, Bangalore 560065, India.
authors.forename: Sandhya
authors.initials: S
authors.lastname: Bhatia
authors.affiliation: Department of Chemistry and Biochemistry, Center for Biomolecular Structure and Organization, University of Maryland , College Park, Maryland 20742, United States.
authors.forename: David
authors.initials: D
authors.lastname: Fushman
authors.affiliation: Department of Biotechnology, Bhupat & Jyoti Mehta School of Biosciences, Indian Institute of Technology Madras (IITM) , Chennai 600036, India.
authors.forename: Athi N
authors.identifier: 0000-0002-1655-7802
authors.initials: AN
authors.lastname: Naganathan
mesh_terms: D001426:Bacterial Proteins / Q000737:chemistry*; D004268:DNA-Binding Proteins / Q000737:chemistry; D029968:Escherichia coli Proteins / Q000737:chemistry; D008954:Models, Biological; D011487:Protein Conformation; D013696:Temperature; D013816:Thermodynamics
chemical_list: D001426:Bacterial Proteins; D004268:DNA-Binding Proteins; D029968:Escherichia coli Proteins; C079209:hha protein, E coli