The G protein-coupled receptor (GPCR) superfamily represents the most important class of pharmaceutical targets. Therefore, the characterization of receptor cascades and their ligands is a prerequisite to discovering novel drugs. Quantification of agonist-induced second messengers and downstream-coupled kinase activities is central to characterization of GPCRs or other pathways that converge on GPCR-mediated signaling. Furthermore, there is a need for simple, cell-based assays that would report on direct or indirect actions on GPCR-mediated effectors of signaling. More generally, there is a demand for sensitive assays to quantify alterations of protein complexes in vivo. We describe the development of a Renilla luciferase (Rluc)-based protein fragment complementation assay (PCA) that was designed specifically to investigate dynamic protein complexes. We demonstrate these features for GPCR-induced disassembly of protein kinase A (PKA) regulatory and catalytic subunits, a key effector of GPCR signaling. Taken together, our observations show that the PCA allows for direct and accurate measurements of live changes of absolute values of protein complex assembly and disassembly as well as cellular imaging and dynamic localization of protein complexes. Moreover, the Rluc-PCA has a sufficiently high signal-tobackground ratio to identify endogenously expressed G␣s proteincoupled receptors. We provide pharmacological evidence that the phosphodiesterase-4 family selectively down-regulates constitutive ␤-2 adrenergic-but not vasopressin-2 receptor-mediated PKA activities. Our results show that the sensitivity of the Rluc-PCA simplifies the recording of pharmacological profiles of GPCR-based candidate drugs and could be extended to high-throughput screens to identify novel direct modulators of PKA or upstream components of GPCR signaling cascades.G protein-coupled receptor ͉ complementation assays ͉ protein-protein interactions ͉ protein fragment G protein-coupled receptors (GPCRs) represent the largest family of cell-surface molecules involved in signal transmission. GPCRs play roles in a broad range of biological processes through regulating the majority of cell-to-cell and cell-toenvironment communication, and, consequently, their dysfunction manifests in numerous diseases (1, 2). The GPCR family has enormous pharmacological importance, as demonstrated by the fact that Ͼ30% of approved drugs elicit their therapeutic effect by selectively acting on known members of this family (3). The human genome harbors Ͼ800 putative GPCRs including a considerable number with unknown physiological function or ligands. GPCR cascades hence remain a major focus of molecular pharmacology (4, 5).Signal transduction by GPCRs is mediated by activation of protein kinases (4), among which the most intensively studied is the cAMP-dependent protein kinase A (PKA) (6). Various extracellular signals converge on the cAMP/PKA pathway through ligand binding to GPCRs. The adenylyl cyclase then converts ATP to the ubiquitous second messenger cAMP. Intrac...

A study was conducted to determine the implication of nitrate reductase (NR) and glutamine synthetase (GS) during the transition of micropropagated plantlets from heterotrophy to photoautotrophy to document how nitrogen metabolism interfaces with photosynthetic and anaplerotic CO2 fixation. The activity of the two enzymes was determined in different tissues at different organogenic stages during the development of plantlets transferred onto rooting media containing varying quantities of sucrose. Under 3% sucrose, NR activity was much higher in leaves than in crown tissues. When roots are initiating, there is a shift in the proportion of nitrate reduction from leaves to crown. As roots mature, the proportion of nitrate reduction increases in roots. Similar trends were observed under 5% sucrose. In contrast, under 1% sucrose, a higher proportion of the nitrate is reduced in the leaf tissues throughout the culture period. This suggests that nitrate is reduced mainly in leaves in photoautotrophic plantlets, while it is reduced in crowns and root tissues for mixotrophic plantlets. In general, the GS activity follows the pattern of NR, but is always in excess, to enable rapid assimilation of ammonium derived from metabolism and medium absorption.