Histidine kinases are sophisticated molecular sensors that are used by bacteria to detect and respond to a multitude of environmental signals. this information to propose a model for the structure of the N-terminal sensor module of KinA. INTRODUCTION Histidine kinases (HKs) are the most ubiquitous molecular sensors used by bacteria. They work in concert with a cognate response regulator (RR) to sense and respond to a plethora of environmental stimuli including changes in pH light temperature cellular energy levels redox state and the presence of toxins and food (1 2 Some HKs are essential for bacterial viability due to Procoxacin their role in essential cellular processes while others are important for mediating antibiotic resistance and virulence; this has led to the idea that some HKs might be good antimicrobial targets (2-5). HKs function by autophosphorylating on a conserved histidine residue and then transferring the resultant high-energy phosphate to a conserved aspartate residue on the RR (6 7 The RR is usually (but not always) a transcription factor that displays altered or enhanced affinity for its cognate DNA recognition elements upon phosphorylation (1). HKs are modular homodimeric proteins. The cytoplasmic C-terminal domain of the proteins is well known bioinformatically as the HisKA site. It is always involved in dimerization autophosphorylation and phosphate transfer and is made up of a four-helix bundle (the dimerization and histidine phosphotransfer [DHp] domain) that carries the phosphorylatable histidine and a C-terminal catalytic domain (often termed “Cat”) which binds ATP (8-10). HisKA is preceded by an N-terminal “sensor” module that varies in length and domain complexity between different HKs (11). Most HKs are membrane bound and the body of the sensor module is typically separated from the catalytic domain by the membrane and the membrane-spanning regions of the protein. There are several HKs however that are entirely cytoplasmic and others that are membrane bound with both their N-terminal sensor and C-terminal catalytic modules in the cytoplasm. The most common cytoplasmic signaling domains are PAS domains (12 13 These domains are found in combination with a great variety of other signaling Procoxacin domains in both plant and animal proteins but in bacteria they are almost exclusively associated with HKs. PAS domains often mediate protein-protein interactions and this function in turn is often modulated via ligand binding to the PAS domain (14-16). PAS domains have been shown to bind a diverse array of ligands including heme flavins 4 acid carboxylic acids and divalent metal ions (17). Sporulation of is a major developmental step that occurs upon nutrient starvation. Whether or not the cell commits to sporulation is determined by the level of phosphorylated Spo0A a master transcription regulator (18 19 which in turn is governed by a complex phosphorelay (20) initiated primarily by autophosphorylation of KinA a cytoplasmic HK. One way in which the phosphorelay is controlled is through regulation of KinA activity via a number of antikinases; these proteins include Sda and KipI both Procoxacin of which block KinA autophosphorylation (21-26). There is also a causal link between the cellular level of KinA and the bacterium’s sporulation status (27). KinA is an unusual HK in that as well as being non-membrane bound its N-terminal sensor module is comprised of three tandem PAS domains termed PASA PASB and PASC (13 28 It was suggested that the sensor module of KinA detects Procoxacin a sporulation-specific signal that regulates the activity of the autokinase (AK) domain. Although this hypothesis cannot be discounted as a mechanism for fine-tuning of KinA function (29) it was recently shown that the sensor module is not essential for KinA activity as it can be substituted with a chimeric construct that supports both KinA multimer formation and host cell sporulation (30). This shows that the N-terminal area of KinA doesn’t have to identify a sporulation Rabbit Polyclonal to TOP2A (phospho-Ser1106). sign to be able to activate KinA which it instead takes on a mainly structural part by improving KinA dimerization which in turn enables autophosphorylation (31). To get this the KinA catalytic site by itself will not Procoxacin travel sporulation nonetheless it allows sporulation when tagged with parts of the N-terminal sensor component that support multimer development (32). Although an purchase of affinity for the putative PAS-PAS homodimer relationships in the KinA sensor continues to be proposed (32).