We describe a bacterial two-hybrid system that allows an easy in vivo screening and selection of functional interactions between two proteins. This genetic test is based on the reconstitution, in an Escherichia coli cya strain, of a signal transduction pathway that takes advantage of the positive control exerted by cAMP. Two putative interacting proteins are genetically fused to two complementary fragments, T25 and T18, that constitute the catalytic domain of Bordetella pertussis adenylate cyclase. Association of the two-hybrid proteins results in functional complementation between T25 and T18 fragments and leads to cAMP synthesis. Cyclic AMP then triggers transcriptional activation of catabolic operons, such as lactose or maltose, that yield a characteristic phenotype. In this genetic test, the involvement of a signaling cascade offers the unique property that association between the hybrid proteins can be spatially separated from the transcriptional activation readout. This permits a versatile design of screening procedures either for ligands that bind to a given ''bait,'' as in the classical yeast two-hybrid system, or for molecules or mutations that block a given interaction between two proteins of interest.

Formation of the Escherichia coli division septum is catalyzed by a number of essential proteins (named Fts) that assemble into a ring-like structure at the future division site. Several of these Fts proteins are intrinsic transmembrane proteins whose functions are largely unknown. Although these proteins appear to be recruited to the division site in a hierarchical order, the molecular interactions underlying the assembly of the cell division machinery remain mostly unspecified. In the present study, we used a bacterial two-hybrid system based on interaction-mediated reconstitution of a cyclic AMP (cAMP) signaling cascade to unravel the molecular basis of septum assembly by analyzing the protein interaction network among E. coli cell division proteins. Our results indicate that the Fts proteins are connected to one another through multiple interactions. A deletion mapping analysis carried out with two of these proteins, FtsQ and FtsI, revealed that different regions of the polypeptides are involved in their associations with their partners. Furthermore, we showed that the association between two Fts hybrid proteins could be modulated by the coexpression of a third Fts partner. Altogether, these data suggest that the cell division machinery assembly is driven by the cooperative association among the different Fts proteins to form a dynamic multiprotein structure at the septum site. In addition, our study shows that the cAMP-based two-hybrid system is particularly appropriate for analyzing molecular interactions between membrane proteins.In Escherichia coli, the cell division process, also referred to as cytokinesis, constriction, or septation, is one of the most central yet poorly understood aspects of the bacterial physiology (for reviews, see references 5, 33, and 45). The event takes place at the midcell and starts after the bacterial chromosomal DNA has been duplicated and segregated into two daughter nucleoids. Cell division genes, named fts, have been identified mainly through conditional mutants that form long filamentous cells at nonpermissive temperatures (4, 5). At present, at least fourteen proteins are known to be specifically required for the E. coli cell septation (for reviews, see references 1, 5, 33, 42, and 45). The majority of the Fts proteins are anchored to the cell membrane, and most of them appear to localize to the bacterial septum in a sequential order (for reviews, see references 5, 33, 35, and 40). Fluorescence microscopy studies using immunofluorescence or the green fluorescent protein (GFP) fused to the Fts proteins have revealed that assembly of the septum starts with the positioning of an FtsZ ring in the cell center. The FtsZ ring is stabilized by FtsA and ZipA, which localize to the septum independently of each other but only in the presence of the FtsZ protein. FtsQ follows FtsK, whose localization requires both FtsA and ZipA proteins, in this hierarchical assembly. Then FtsL, FtsB, FtsW, FtsI, FtsN, and AmiC are successively recruited to the FtsZ ring (for reviews, see refere...

No abstract

The GraSR two-component system (TCS) controls cationic antimicrobial peptide (CAMP) resistance in Staphylococcus aureus through the synthesis of enzymes that increase bacterial cell surface positive charges, by D-alanylation of teichoic acids and lysylination of phosphatidylglycerol, leading to electrostatic repulsion of CAMPs. The GraS histidine kinase belongs to the "intramembrane-sensing kinases" subfamily, with a structure featuring a short amino-terminal sensing domain, and two transmembrane helices separated only by a short loop, thought to be buried in the cytoplasmic membrane. The GraSR TCS is in fact a multicomponent system, requiring at least one accessory protein, GraX, in order to function, which, as we show here, acts by signaling through the GraS kinase. The graXRS genes are located immediately upstream from genes encoding an ABC transporter, vraFG, whose expression is controlled by GraSR. We demonstrated that the VraFG transporter does not act as a detoxification module, as it cannot confer resistance when produced on its own, but instead plays an essential role by sensing the presence of CAMPs and signaling through GraS to activate GraR-dependent transcription. A bacterial two-hybrid approach, designed to identify interactions between the GraXSR and VraFG proteins, was carried out in order to understand how they act in detecting and signaling the presence of CAMPs. We identified many interactions between these protein pairs, notably between the GraS kinase and both GraX and the VraG permease, indicating the existence of an original five-component system involved in CAMP sensing and signal transduction to promote S. aureus resistance.