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We investigated which subtypes of G-protein beta subunits participate in voltage-dependent modulation of N-type calcium channels. Calcium currents were recorded from cultured rat superior cervical ganglion neurons injected intranuclearly with DNA encoding five different G-protein beta subunits. Gbeta1 and Gbeta2 strongly mimicked the fast voltage-dependent inhibition of calcium channels produced by many G-protein-coupled receptors. The Gbeta5 subunit produced much weaker effects than Gbeta1 and Gbeta2, whereas Gbeta3 and Gbeta4 were nearly inactive in these electrophysiological studies. The specificity implied by these results was confirmed and extended using the yeast two-hybrid system to test for protein-protein interactions. Here, Gbeta1 or Gbeta2 coupled to the GAL4-activation domain interacted strongly with a channel sequence corresponding to the intracellular loop connecting domains I and II of a alpha1 subunit of the class B calcium channel fused to the GAL4 DNA-binding domain. In this assay, the Gbeta5 subunit interacted weakly, and Gbeta3 and Gbeta4 failed to interact. Together, these results suggest that Gbeta1 and/or Gbeta2 subunits account for most of the voltage-dependent inhibition of N-type calcium channels and that the linker between domains I and II of the calcium channel alpha1 subunit is a principal receptor for this inhibition.

We have determined the relative abilities of several members of the G protein ␤ and ␥ subunit families to associate with each other using the yeast two-hybrid system. We show first that the mammalian ␤1 and ␥3 fusion proteins form a complex in yeast and that formation of the complex activates the reporter gene for ␤-galactosidase. Second, the magnitude of reporter activity stimulated by various combinations of ␤ and ␥ subunit types varies widely. Third, the reporter activity evoked by a particular combination of ␤ and␥ subunit types is not correlated with the expression levels of these subunit types in the yeast cells. Finally, the reporter activity shows a direct relationship with the amount of hybrid ␤␥ complex formed in the cell as determined by immunoprecipitation. These results suggest that different ␤ and ␥ subunit types interact with each other with widely varying abilities, and this in combination with the level of expression of a subunit type in a mammalian cell determines which G protein will be active in that cell. The strong preference of all ␥ subunit types for the ␤1 subunit type explains the preponderence of this subunit type in most G proteins.Most of the neurohormonal signaling pathways in mammals are mediated by heterotrimeric G proteins (1, 2). Most cells contain many different G protein subunit types, and yet particular agonists evoke a highly specific response in a cell by activating a defined G protein-mediated signaling pathway (3). Various mechanisms can contribute to this specificity. For instance, in a cell that contains many different ␣, ␤, and ␥ subunit types, only certain types may be capable of forming a heterotrimeric complex because of the differences in the intrinsic affinity of these subunit types for one another. It has been shown that interactions between two different ␥ subunit types (␥1 and ␥2) and three different ␤ subunit types (␤1-␤3) are selective, indicating that this is indeed a mechanism for achieving specificity (4, 5). However, these experiments were performed using in vivo or in vitro systems that could detect differences in the ability of these subunit types to interact but were not sufficiently sensitive to detect low level interaction between some of the subunit types. To obtain a more sensitive measure of the interaction between various members of the ␤ and ␥ subunit families, we used the yeast two-hybrid system. In this system, the proteins with the potential to interact are expressed as hybrids of two different domains of a transcription factor. If the proteins interact with each other, the transcription factor domains are in proximity and capable of activating a promoter that controls reporter activity (6). We chose this system because it measures protein-protein interaction in a yeast cell and is therefore a reasonably accurate reflection of the ability of the interacting proteins to form a complex in a cell. Furthermore, it is highly sensitive in comparison with the methods used before to measure protein-protein interaction. For instance, in an assay of in...

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Retapamulin is a semisynthetic pleuromutilin derivative being developed as a topical antibiotic for treating bacterial infections of the skin. It is potent in vitro against susceptible and multidrug-resistant organisms commonly associated with bacterial skin infections. We report detailed mode of action studies demonstrating that retapamulin binds to the bacterial ribosome with high affinity, inhibits ribosomal peptidyl transferase activity, and partially inhibits the binding of the initiator tRNA substrate to the ribosomal P-site. Taken together, these data distinguish the mode of action of retapamulin from that of other classes of antibiotics. This unique mode of action may explain the lack of clinically relevant, target-specific cross-resistance of retapamulin with antibacterials in current use.Pleuromutilin is a tricyclic, diterpene natural product first identified from the basidiomycete bacterial species Pleurotus mutilus (now referred to as Clitopilus scyphoides) that possesses modest antibacterial activity against primarily gram-positive bacterial organisms (8). Early studies on the mode of action of tiamulin and other semisynthetic pleuromutilin derivatives showed that it interfered with cell-free protein synthesis and formylmethionine-puromycin synthesis (5). Further work has shown that the binding of these compounds to the bacterial 50S ribosomal subunit is displaced by puromycin and chloramphenicol (6). Competitive rRNA footprinting experiments also indicated that tiamulin and another analogue, valnemulin, can bind concurrently with the macrolide antibiotic erythromycin. In contrast, these compounds compete with the peptidyl transferase inhibitor carbomycin (13). Additional evidence for the binding of pleuromutilins to the peptidyl transferase center has come from recent x-ray crystallographic data (17) which show tiamulin having interactions with both the ribosomal A-and P-sites. The interaction of pleuromutilins also appears to involve ribosomal protein L3 near the peptidyl transferase center, since Brachyspira sp. isolates with reduced tiamulin susceptibility have recently been identified with mutations in L3 (14). Thus, it is becoming clear that pleuromutilins have a mechanism of action which involves nucleotides within the peptidyl transferase center.Despite the discovery and development of tiamulin and valnemulin, which have become important veterinary agents to treat swine disease, there has been little progress in the identification of pleuromutilin derivatives to treat bacterial infections in humans. Hence, there has been a renewed interest in the exploitation of novel pleuromutilin derivatives for deployment in human clinical practice (2,7,20). Retapamulin (Fig. 1A) is a selective, prokaryotic protein synthesis inhibitor being developed as a topical antibiotic for treating bacterial infections of the skin. This semisynthetic pleuromutilin has potent in vitro activity against susceptible and multidrug-resistant organisms commonly associated with bacterial skin infections (15, 16). Furthermor...

The G protein ␤␥ complex modulates the function of a variety of effectors in biological signaling. However, the individual roles of the ␤ and ␥ subunits in this interaction are unknown. Unlike in the case of the ␣ subunit, domains on the ␤␥ complex that contact effectors have not yet been identified. We show here using the yeast two-hybrid system that the ␤ subunit and not the ␥ subunit interacts with domains specific to adenylyl cyclase type 2 (AC2) and the muscarinic receptor-gated atrial inwardly rectifying potassium channel, GIRK1. Different ␤ subunit types interact with these effector domains with different efficacies. Furthermore, an N-terminal fragment of 100 residues interacts with both these effector domains as effectively as the whole ␤ subunit. This domain includes the region where the ␤ subunit contacts with the ␣ subunit in the crystal structure and may therefore explain the ability of the ␣ subunit to shut off the activity of the ␤␥ complex.Both the ␣ subunit and the ␤␥ complex of a heterotrimeric G protein are capable of regulating the function of a variety of effectors (1). Among the effectors regulated by the ␤␥ complex are adenylyl cyclases, phospholipase C-␤, and potassium channels (2-6). The ␤␥ complex also interacts with the ␤-adrenergic receptor kinase (7,8). Several studies have now shown that the ␤␥ complex acts on these molecules directly (9 -16). Since the evidence so far indicates that the ␤ and ␥ subunits are tightly bound as a complex, it has not been clear whether one or both subunits interact with effectors. Several other questions have also remained. What are the relative positions of sites on ␤␥ that interact with unrelated effectors such as adenylyl cyclases and potassium channels? What is the mechanism that allows ␤␥ to be switched off after acting on an effector? Is there specificity of interaction between ␤␥ subtypes and effectors since both the ␤ and ␥ subunits are families of proteins which are capable of combining to form a variety of complexes (17, 18)?.We have recently shown that G protein ␤ and ␥ subunit types effectively interact with each other as fusion proteins in the yeast two-hybrid system (19). These studies also showed that the reporter activity elicited by particular combinations of ␤ and ␥ subunit types was directly related to the amount of that complex formed in the cell as determined by immunoprecipitation. We therefore used the two-hybrid system to examine interaction of the ␤␥ complex with effectors. We chose to analyze interactions of ␤␥ with adenylyl cyclase type 2 (AC2) and the muscarinic receptor-gated atrial inwardly rectifying potassium channel (GIRK1) since domains on these effectors which interact with ␤␥ had been identified (11)(12)(13)(14)20). A 27-residue peptide specific to the second of the two large cytoplasmic loops of AC2 was shown to be capable of inhibiting the ability of the ␤␥ complex to regulate several effectors, AC2, AC1, K ϩ channels, phospholipase C-␤, and the ␤-adrenergic receptor kinase (20). These results indicated that a single...

A fluorescence polarization assay is described that measures the binding of fluorescently labeled erythromycin to 70S ribosomes from Escherichia coli and the displacement of the erythromycin from these ribosomes. The assay has been validated with several macrolide derivatives and other known antibiotics. We demonstrate that this assay is suitable for determining the dissociation constants of novel compounds that have binding sites overlapping those of macrolides. This homogeneous binding assay provides a valuable tool for defining structure-activity relationships among compounds during the discovery and development of new ribosometargeting drugs.Macrolide antibiotics comprise a large group of clinically useful compounds, characterized by having a 14-, 15-, or 16-membered lactone ring with two or more sugar groups attached. Of particular importance are the 14-membered macrolide erythromycin and its newer-generation derivatives clarithromycin, roxithromycin, and azithromycin (a 15-membered macrolide), which are valuable therapeutic agents for the treatment of community-acquired respiratory tract infections. These antibiotics selectively inhibit bacterial protein synthesis by binding reversibly to the ribosome. They have been shown to interact with domain V of 23S rRNA near the peptidyl transferase center (8). Recent structural work (12, 21) has confirmed that these antibiotics exert their inhibitory effect by blocking the entrance to the polypeptide exit tunnel on the 50S ribosomal subunit and thus preventing the extrusion of nascent polypeptides. Sixteen-membered macrolides, such as tylosin, spiramycin, and carbomycin, also bind so as to block the peptide exit tunnel, but in addition these antibiotics have a mycaminose-mycarose disaccharide side chain that protrudes towards the peptidyl transferase center and more directly inhibit the peptidyl transferase reaction (12).Since the introduction of erythromycin nearly 50 years ago, antibacterial resistance to macrolides has been an increasingly persistent threat to public health. One major resistance mechanism is a form of target modification, wherein the dimethylation of A2058 of the 23S rRNA by erm gene-encoded ribosomal methylases results in cross-resistance to macrolides, to the structurally related lincosamides, and to group B streptogramins (commonly referred to as the MLS B phenotype). The emergence of these resistant bacterial pathogens has been of particular concern and has fueled the search for newer and more potent antibiotics with activity against these organisms.For years the interaction of erythromycin and other macrolides with the bacterial ribosome has been the focus of intense investigation, and a number of experimental approaches have been developed to characterize the equilibria of binding and the kinetics of these drugs with their ribosomal target. In many of these studies, the binding of macrolides to ribosomes was investigated using a radiolabeled antibiotic as a ligand (6,7,10,11,19,22). Since binding was measured by filtration, RNA footprintin...

Gram-negative bacteria have evolved numerous two-component systems (TCSs) to cope with external environmental changes. The CpxA/CpxR TCS consisting of the kinase CpxA and the regulator CpxR, is known to be involved in the biofilm formation and virulence of Escherichia coli. However, the role of CpxA/CpxR remained unclear in Actinobacillus pleuropneumoniae, a bacterial pathogen that can cause porcine contagious pleuropneumonia (PCP). In this report, we show that CpxA/CpxR contributes to the biofilm formation ability of A. pleuropneumoniae. Furthermore, we demonstrate that CpxA/CpxR plays an important role in the expression of several biofilm-related genes in A. pleuropneumoniae, such as rpoE and pgaC. Furthermore, The results of electrophoretic mobility shift assays (EMSAs) and DNase I footprinting analysis demonstrate that CpxR-P can regulate the expression of the pgaABCD operon through rpoE. In an experimental infection of mice, the animals infected with a cpxA/cpxR mutant exhibited delayed mortality and lower bacterial loads in the lung than those infected with the wildtype bacteria. In conclusion, these results indicate that the CpxA/CpxR TCS plays a contributing role in the biofilm formation and virulence of A. pleuropneumoniae.