Structural Insights into Sporulation in Bacillus sibtilis

PAS domains are modular domains that providing specific interaction surfaces for a diverse array of ligands, from small organic compounds to intra and intermolecular protein domains. As such, they are ubiquitous throughout signal transduction pathways in all three kingdoms of life: Bacteria, Archae, and Eucarya (Huang, Edery et al. 1993). The significant role played by PAS domains in Bacillus subtilis sporulation is underscored by the finding that the PAS-A domain is necessary for efficient phosphorylation in the sporulation kinase, KinA(Wang, Fabret et al. 2001). This activity is necessary for initiating a phosphorelay that results in the upregulation of genes required for sporulation. In KinA, dimerization is necessary for individual monomers of the histidine kinase domain to transphosphorylate partner subunits. The dimerization of KinA involves interactions in the PAS-A domain, but molecular details regarding PAS-A dimerization and its importance to KinA activity has not been previous characterized. To investigate these interactions within the context of the KinA homodimer, we expressed the N-terminal PAS-A domain and solved the X-ray crystal structure. Conformational variability was implicated through the observation of different orientations of the dimerization interface in two distinct structural models found in the asymmetric unit of the crystal lattice. These models were used to identify key interfacial residues and the roles of these were tested in a variety of ways by site-directed mutagenesis.