Isolation and characterization of constitutively active mutants of mammalian adenylyl cyclase

Gsα stimulation of mammalian adenylate cyclases regulated by their hexahelical membrane anchors

Mammalian adenylate cyclases (ACs) are pseudoheterodimers with dissimilar hexahelical membrane-anchors, isoform-specifically conserved for more than half a billion years. We exchanged both membrane anchors of the AC isoform 2 by the quorum-sensing receptor from Vibrio harveyi, CqsS, which has a ligand, Cholera-Autoinducer-1 (CAI-1). In the chimera, AC activity was stimulated by Gsα, CAI-1 had no effect. Surprisingly, CAI-1 inhibited Gsα stimulation. We report that Gsα stimulation of human AC isoforms 2, 3, 5, and 9 expressed in Sf9 cells is inhibited by serum as is AC activity in membranes isolated from rat brain cortex. AC2 activation by forskolin or forskolin/Gsα was similarly inhibited. Obviously, serum contains as yet unidentified factors affecting AC activity. The data establish a linkage in ACs, in which the membrane anchors, as receptors, transduce extracellular signals to the cytosolic catalytic dimer. A mechanistic three state model of AC regulation is presented compatible with all known regulatory inputs into mammalian ACs. The data allow designating the membrane anchors of mammalian ACs as orphan receptors, and establish a new level of AC regulation.

Synergistic mutations in soluble guanylyl cyclase (sGC) reveal a key role for interfacial regions in the sGC activation mechanism

Soluble guanylyl cyclase (sGC) is the main receptor for nitric oxide (NO) and a central component of the NO-cGMP pathway, critical to cardiovascular function. NO binding to the N-terminal sensor domain in sGC enhances the cyclase activity of the C-terminal catalytic domain. Our understanding of the structural elements regulating this signaling cascade is limited, hindering structure-based drug design efforts that target sGC to improve the management of cardiovascular diseases. Conformational changes are thought to propagate the NO-binding signal throughout the entire sGC heterodimer, via its coiled-coil domain, to reorient the catalytic domain into an active conformation. To identify the structural elements involved in this signal transduction cascade, here we optimized a cGMP-based luciferase assay that reports on heterologous sGC activity in Escherichia coli and identified several mutations that activate sGC. These mutations resided in the dorsal flaps, dimer interface, and GTP-binding regions of the catalytic domain. Combinations of mutations from these different elements synergized, resulting in even greater activity and indicating a complex cross-talk among these regions. Molecular dynamics simulations further revealed conformational changes underlying the functional impact of these mutations. We propose that the interfacial residues play a central role in the sGC activation mechanism by coupling the coiled-coil domain to the active site via a series of hot spots. Our results provide new mechanistic insights not only into the molecular pathway for sGC activation but also for other members of the larger nucleotidyl cyclase family.

Structure/function of the soluble guanylyl cyclase catalytic domain

Soluble guanylyl cyclase (GC-1) is the primary receptor of nitric oxide (NO) in smooth muscle cells and maintains vascular function by inducing vasorelaxation in nearby blood vessels. GC-1 converts guanosine 5′-triphosphate (GTP) into cyclic guanosine 3′,5′-monophosphate (cGMP), which acts as a second messenger to improve blood flow. While much work has been done to characterize this pathway, we lack a mechanistic understanding of how NO binding to the heme domain leads to a large increase in activity at the C-terminal catalytic domain. Recent structural evidence and activity measurements from multiple groups have revealed a low-activity cyclase domain that requires additional GC-1 domains to promote a catalytically-competent conformation. How the catalytic domain structurally transitions into the active conformation requires further characterization. This review focuses on structure/function studies of the GC-1 catalytic domain and recent advances various groups have made in understanding how catalytic activity is regulated including small molecules interactions, Cys-S-NO modifications and potential interactions with the NO-sensor domain and other proteins.

High-throughput FACS-based mutant screen identifies a gain-of-function allele of the Fusarium graminearum adenylyl cyclase causing deoxynivalenol over-production

Fusarium head blight and crown rot, caused by the fungal plant pathogen Fusarium graminearum, impose a major threat to global wheat production. During the infection, plants are contaminated with mycotoxins such as deoxynivalenol (DON), which can be toxic for humans and animals. In addition, DON is a major virulence factor during wheat infection. However, it is not fully understood how DON production is regulated in F. graminearum. In order to identify regulators of DON production, a high-throughput mutant screen using Fluorescence Activated Cell Sorting (FACS) of a mutagenised TRI5-GFP reporter strain was established and a mutant over-producing DON under repressive conditions identified. A gain-of-function mutation in the F. graminearum adenylyl cyclase (FAC1), which is a known positive regulator of DON production, was identified as the cause of this phenotype through genome sequencing and segregation analysis. Our results show that the high-throughput mutant screening procedure developed here can be applied for identification of fungal proteins involved in diverse processes.

Interfacial residues promote an optimal alignment of the catalytic center in human soluble guanylate cyclase: heterodimerization is required but not sufficient for activity

Soluble guanylate cyclase (sGC) plays a central role in the cardiovascular system and is a drug target for the treatment of pulmonary hypertension. While the three-dimensional structure of sGC is unknown, studies suggest that binding of the regulatory domain to the catalytic domain maintains sGC in an autoinhibited basal state. The activation signal, binding of NO to heme, is thought to be transmitted via the regulatory and dimerization domains to the cyclase domain and unleashes the full catalytic potential of sGC. Consequently, isolated catalytic domains should show catalytic turnover comparable to that of activated sGC. Using X-ray crystallography, activity measurements, and native mass spectrometry, we show unambiguously that human isolated catalytic domains are much less active than basal sGC, while still forming heterodimers. We identified key structural elements regulating the dimer interface and propose a novel role for residues located in an interfacial flap and a hydrogen bond network as key modulators of the orientation of the catalytic subunits. We demonstrate that even in the absence of the regulatory domain, additional sGC domains are required to guide the appropriate conformation of the catalytic subunits associated with high activity. Our data support a novel regulatory mechanism whereby sGC activity is tuned by distinct domain interactions that either promote or inhibit catalytic activity. These results further our understanding of heterodimerization and activation of sGC and open additional drug discovery routes for targeting the NO–sGC–cGMP pathway via the design of small molecules that promote a productive conformation of the catalytic subunits or disrupt inhibitory domain interactions.

The Dunce cAMP phosphodiesterase PDE-4 negatively regulates G alpha(s)-dependent and G alpha(s)-independent cAMP pools in the Caenorhabditis elegans synaptic signaling network

Forward genetic screens for mutations that rescue the paralysis of ric-8 (Synembryn) reduction-of-function mutations frequently reveal mutations that cause hyperactivation of one or more components of the G alpha(s) pathway. Here, we report that one of these mutations strongly reduces the function of the Dunce cAMP phosphodiesterase PDE-4 by disrupting a conserved active site residue. Loss of function and neural overexpression of PDE-4 have profound and opposite effects on locomotion rate, but drug-response assays suggest that loss of PDE-4 function does not affect steady-state acetylcholine release or reception. Our genetic analysis suggests that PDE-4 regulates both G alpha(s)-dependent and G alpha(s)-independent cAMP pools in the neurons controlling locomotion rate. By immunostaining, PDE-4 is strongly expressed throughout the nervous system, where it localizes to small regions at the outside boundaries of synaptic vesicle clusters as well as intersynaptic regions. The synaptic subregions containing PDE-4 are distinct from those containing active zones, as indicated by costaining with an antibody against the long form of UNC-13. This highly focal subsynaptic localization suggests that PDE-4 may exert its effects by spatially regulating intrasynaptic cAMP pools.

Mutations that rescue the paralysis of Caenorhabditis elegans ric-8 (synembryn) mutants activate the G alpha(s) pathway and define a third major branch of the synaptic signaling network

To identify hypothesized missing components of the synaptic G alpha(o)-G alpha(q) signaling network, which tightly regulates neurotransmitter release, we undertook two large forward genetic screens in the model organism C. elegans and focused first on mutations that strongly rescue the paralysis of ric-8(md303) reduction-of-function mutants, previously shown to be defective in G alpha(q) pathway activation. Through high-resolution mapping followed by sequence analysis, we show that these mutations affect four genes. Two activate the G alpha(q) pathway through gain-of-function mutations in G alpha(q); however, all of the remaining mutations activate components of the G alpha(s) pathway, including G alpha(s), adenylyl cyclase, and protein kinase A. Pharmacological assays suggest that the G alpha(s) pathway-activating mutations increase steady-state neurotransmitter release, and the strongly impaired neurotransmitter release of ric-8(md303) mutants is rescued to greater than wild-type levels by the strongest G alpha(s) pathway activating mutations. Using transgene induction studies, we show that activating the G alpha(s) pathway in adult animals rapidly induces hyperactive locomotion and rapidly rescues the paralysis of the ric-8 mutant. Using cell-specific promoters we show that neuronal, but not muscle, G alpha(s) pathway activation is sufficient to rescue ric-8(md303)'s paralysis. Our results appear to link RIC-8 (synembryn) and a third major G alpha pathway, the G alpha(s) pathway, with the previously discovered G alpha(o) and G alpha(q) pathways of the synaptic signaling network.

Functional characterization of nitric oxide and YC-1 activation of soluble guanylyl cyclase: structural implication for the YC-1 binding site?

Soluble guanylyl cyclase (sGC) is a heterodimeric enzyme formed by an alpha subunit and a beta subunit, the latter containing the heme where nitric oxide (NO) binds. When NO binds, the basal activity of sGC is increased several hundred fold. sGC activity is also increased by YC-1, a benzylindazole allosteric activator. In the presence of NO, YC-1 synergistically increases the catalytic activity of sGC by enhancing the affinity of NO for the heme. The site of interaction of YC-1 with sGC is unknown. We conducted a mutational analysis to identify the binding site and to determine what residues were involved in the propagation of NO and/or YC-1 activation. Because guanylyl cyclases (GCs) and adenylyl cyclases (ACs) are homologous, we used the three-dimensional structure of AC to guide the mutagenesis. Biochemical analysis of purified mutants revealed that YC-1 increases the catalytic activity not only by increasing the NO affinity but also by increasing the efficacy of NO. Effects of YC-1 on NO affinity and efficacy were dissociated by single-point mutations implying that YC-1 has, at least, two types of interaction with sGC. A structural model predicts that YC-1 may adopt two configurations in one site that is pseudosymmetric with the GTP binding site and equivalent to the forskolin site in AC.

Functional analysis of the interface regions involved in interactions between the central cytoplasmic loop and the C-terminal tail of adenylyl cyclase

The mammalian adenylyl cyclase is a membrane-bound enzyme that is predicted to have 12 trans-membrane spans. Between membrane spans 6 and 7 there is a large cytoplasmic loop, which, along with the C-terminal tail, makes up the catalytic site of the enzyme. Crystal structures of these soluble cytoplasmic domains have identified the regions that are involved in interactions with each other. The functional consequences of these interactions in the full-length membrane-embedded enzymes have not been established. In this study, we analyzed the role of various interaction regions within the central cytoplasmic loop (C1) and the C-terminal tail (C2) on basal, Galphas-, forskolin-, and Mn(2+)-stimulated activities of adenylyl cyclases 2 and 6 (AC2 and AC6). We tested synthetic peptides encoding the different interface surfaces of both the C1 and C2 domain on different activities of membrane-bound AC2 and AC6 expressed in insect cells. We found the C1-alpha2-beta2-beta3 and C2-beta2'-beta3' regions to be involved in stimulation by Galphas and forskolin but not in the basal or Mn(2+)-stimulated activities. Both the C1-beta4-beta5-alpha4 region and the C2-alpha3'-beta4' region play a role in the Galphas- and forskolin-stimulated activities as well as in basal activity, because the peptides encoding these regions inhibit basal activity by 30%. In contrast, the C2-alpha2' region peptide inhibits both basal and Mn(2+)-stimulated activity by >50%. These results suggest that the different stimulated activities may involve distinct interface interactions in the intact enzyme and, consequently, the distinct mechanisms by which Mn(2+) activates the enzyme as compared with Galphas and forskolin, leading to the possibility that the full-length adenylyl cyclase may have multiple catalytically competent configurations.

Catalysis of amide synthesis by RNA phosphodiester and hydroxyl groups

The functional groups found among the RNA bases and in the phosphoribose backbone represent a limited repertoire from which to construct a ribozyme active site. This work investigates the possibility that simple RNA phosphodiester and hydroxyl functional groups could catalyze amide bond synthesis. Reaction of amine groups with activated esters would be catalyzed by a group that stabilizes the partial positive charge on the amine nucleophile in the transition state. 2'-Amine substitutions adjacent to 3'-phosphodiester or 3'-hydroxyl groups react efficiently with activated esters to form 2'-amide and peptide products. In contrast, analogs in which the 3'-phosphodiester is replaced by an uncharged phosphotriester or is constrained in a distal conformation react at least 100-fold more slowly. Similarly, a nucleoside in which the 3'-hydroxyl group is constrained trans to the 2'-amine is also unreactive. Catalysis of synthetic reactions by RNA phosphodiester and ribose hydroxyl groups is likely to be even greater in the context of a preorganized and solvent-excluding catalytic center. One such group is the 2'-hydroxyl of the ribosome-bound P-site adenosine substrate, which is close to the amine nucleophile in the peptidyl synthesis reaction. Given ubiquitous 2'-OH groups in RNA, there exists a decisive advantage for RNA over DNA in catalyzing reactions of biological significance.

Mechanism of Galpha i-mediated inhibition of type V adenylyl cyclase

The topology of mammalian adenylyl cyclase reveals an integral membrane protein composed of an alternating series of membrane and cytoplasmic domains (C1 and C2). The stimulatory G protein, Galpha(s), binds within a cleft in the C2 domain of adenylyl cyclase while Galpha(i) binds within the opposite cleft in the C1 domain. The mechanism of these two regulators also appears to be in opposition. Activation of adenylyl cyclase by Galpha(s) or forskolin results in a 100-fold increase in the apparent affinity of the two domains for one another. We show herein that Galpha(i) reduces C1/C2 domain interaction and thus formation of the adenylyl cyclase catalytic site. Mutants that increase the affinity of C1 for C2 decrease the ability of Galpha(i) to inhibit the enzyme. In addition, Galpha(i) can influence binding of molecules to the catalytic site, which resides at the C1/C2 interface. Adenylyl cyclase can bind substrate analogs in the presence of Galpha(i) but cannot simultaneously bind Galpha(i) and transition state analogs such as 2'd3'-AMP. Galpha(i) also cannot inhibit the membrane-bound enzyme in the presence of manganese, which increases the affinity of adenylyl cyclase for ATP and substrate analogs. Thus homologous G protein alpha-subunits promote bidirectional regulation at the domain interface of the pseudosymmetrical adenylyl cyclase enzyme.

Regulation of adenylyl cyclases by a region outside the minimally functional cytoplasmic domains

The highly conserved topological structure of G protein-activated adenylyl cyclases seems unnecessary because the soluble cytoplasmic domains retain regulatory and catalytic properties. Yet, we previously isolated a constitutively active mutant of the Dictyostelium discoideum adenylyl cyclase harboring a single point mutation in the region linking the cytoplasmic and membrane domains (Leu-394). We show here that multiple amino acid substitutions at Leu-394 also display constitutive activity. The constitutive activity of these mutants is not dependent on G proteins or cytosolic regulators, although some of the mutants can be activated to higher levels than wild type. Combining a constitutive mutation such as L394T with K482N, a point mutation that renders the enzyme insensitive to regulators, restores an enzyme with wild type properties of low basal activity and the capacity to be activated by G proteins. Thus regions located outside the cytoplasmic loops of adenylyl cyclases are not only important in the acquisition of an activated conformation, they also have impact on other regions within the catalytic core of the enzyme.

Inhibition of epithelial chloride secretion by butyrate: role of reduced adenylyl cyclase expression and activity

Butyrate and other short-chain fatty acids (SCFAs) are found at high concentrations in the colonic lumen and affect multiple epithelial cell functions. To better understand how SCFAs regulate ion transport, we investigated the effects of SCFAs on Cl(-) secretion in human colonic epithelial cell line T(84). Butyrate inhibited Cl(-) secretory responses to prostaglandin E(2), forskolin, and cholera toxin. Other SCFAs were less effective or inactive. Reduced secretion was associated with decreased synthesis of the second messenger cAMP rather than increased degradation. Expression and activity of adenylyl cyclase were decreased by butyrate, whereas phosphodiesterase activity was unaffected and phosphodiesterase inhibition did not reverse the effects of butyrate on Cl(-) secretion. Furthermore, butyrate decreased expression of the basolateral Na-K-2Cl cotransporter, indicating that it might modulate the secretory capacity of the cells. However, butyrate did not affect secretory responses to the calcium-dependent secretagogue carbachol, cAMP analogs, or uroguanylin, indicating that normal secretory responses to adequate levels of second messengers in butyrate-treated T(84) cells are possible. These results show that butyrate affects several aspects of epithelial Cl(-) secretion, including second messenger generation and expression of key ion transporters. However, these effects may not all be equally important in determining Cl(-) secretion in response to physiologically relevant secretagogues.

An isoform-specific interaction of the membrane anchors affects mammalian adenylyl cyclase type V activity

The nine membrane-bound mammalian adenylyl cyclases (ACs) contain two highly diverged membrane anchors, M1 and M2, with six transmembrane spans each and two conserved cytosolic domains which coalesce into a pseudoheterodimeric catalytic unit. Previously, the catalytic segments, bacterially expressed as soluble proteins, were characterized extensively whereas the function of the membrane domains remained unexplored. Using the catalytic C1 and C2 domains of AC type V we employed the membrane anchors from type V and VII ACs for construction of enzymes with duplicated, inverted, fully swapped and chimeric membrane anchors. Further, in the M1 membrane domain individual transmembrane spans were removed or exchanged between type V and VII ACs. The constructs were expressed in HEK293 cells, the expression levels and membrane localization was assessed by Western blotting. Cell-free basal, forskolin-, GTP gamma S-and G(s alpha)/GTP gamma S-stimulated AC activities were determined. The results demonstrate that enzymatic activities were only maintained when the M1 and M2 membrane domains were derived from either AC V or VII. Constructs with chimeric membrane domains, i.e. M1 from type V and M2 from type VII AC or vice versa, were essentially inactive although the expression levels and membrane localization appeared to be normal. The data indicate a functionally important interaction of the membrane domains of ACs in that they seem to interact in a pair-like, isoform delimited manner. This interaction directly impinges on the formation of the catalytic interface. We propose that protein-protein interactions of the AC membrane domains may constitute another, yet unexplored level of AC regulation.