Immunoassays and Mass Spectrometry for Determination of Protein Concentrations in Genetically Modified Crops

Quantifying protein levels in genetically modified (GM) crops is crucial in every phase of development, deregulation, and seed production. Immunoassays, particularly enzyme-linked immunosorbent assay, have been the primary protein quantitation techniques for decades within the industry due to their efficiency, adaptability, and credibility. Newer immunoassay technologies like Meso Scale Discovery and Luminex offer enhanced sensitivity and multiplexing capabilities. While mass spectrometry (MS) has been widely used for small molecules and protein detection in the pharmaceutical and agricultural industries (e.g., biomarkers, endogenous allergens), its use in quantifying protein levels in GM crops has been limited. However, as trait portfolios for GM crop have expanded, MS has been increasingly adopted due to its comparable sensitivity, increased specificity, and multiplexing capabilities. This review contrasts the benefits and limitations of immunoassays and MS technologies for protein measurement in GM crops, considering factors such as cost, convenience, and specific analytical needs. Ultimately, both techniques are suitable for assessing protein concentrations in GM crops, with MS offering complementary capabilities to immunoassays. This comparison aims to provide insights into selecting between these techniques based on the user's end point needs.

Kinetic scaffolding mediated by a phospholipase C-beta and Gq signaling complex

Transmembrane signals initiated by a broad range of extracellular stimuli converge on nodes that regulate phospholipase C (PLC)-dependent inositol lipid hydrolysis for signal propagation. We describe how heterotrimeric guanine nucleotide-binding proteins (G proteins) activate PLC-βs and in turn are deactivated by these downstream effectors. The 2.7-angstrom structure of PLC-β3 bound to activated Gα(q) reveals a conserved module found within PLC-βs and other effectors optimized for rapid engagement of activated G proteins. The active site of PLC-β3 in the complex is occluded by an intramolecular plug that is likely removed upon G protein-dependent anchoring and orientation of the lipase at membrane surfaces. A second domain of PLC-β3 subsequently accelerates guanosine triphosphate hydrolysis by Gα(q), causing the complex to dissociate and terminate signal propagation. Mutations within this domain dramatically delay signal termination in vitro and in vivo. Consequently, this work suggests a dynamic catch-and-release mechanism used to sharpen spatiotemporal signals mediated by diverse sensory inputs.

Crystal structure of the multifunctional Gbeta5-RGS9 complex

Regulators of G-protein signaling (RGS) proteins enhance the intrinsic GTPase activity of G protein alpha (Galpha) subunits and are vital for proper signaling kinetics downstream of G protein-coupled receptors (GPCRs). R7 subfamily RGS proteins specifically and obligately dimerize with the atypical G protein beta5 (Gbeta5) subunit through an internal G protein gamma (Ggamma)-subunit-like (GGL) domain. Here we present the 1.95-A crystal structure of the Gbeta5-RGS9 complex, which is essential for normal visual and neuronal signal transduction. This structure reveals a canonical RGS domain that is functionally integrated within a molecular complex that is poised for integration of multiple steps during G-protein activation and deactivation.

Molecular mechanism of membrane docking by the Vam7p PX domain

The Vam7p t-SNARE is an essential component of the vacuole fusion machinery that mediates membrane trafficking and protein sorting in yeast. Vam7p is recruited to vacuoles by its N-terminal PX domain that specifically recognizes PtdIns(3)P in the bilayers, however the precise mechanism of membrane anchoring remains unclear. Here we describe a molecular basis for membrane targeting and penetration by the Vam7p PX domain based on structural and quantitative analysis of its interactions with lipids and micelles. Our results derived from in vitro binding measurements using NMR, monolayer surface tension experiments and mutagenesis reveal a multivalent membrane docking mechanism involving specific PtdIns(3)P recognition that is facilitated by electrostatic interactions and accompanying hydrophobic insertion. Both the hydrophobic and electrostatic components enhance the Vam7p PX domain association with PtdIns(3)P-containing membranes. The inserting Val(70), Leu(71), and Trp(75) residues located next to the PtdIns(3)P binding pocket are surrounded by a basic patch, which is involved in nonspecific electrostatic contacts with acidic lipids, such as PtdSer. Substitution of the insertion residues significantly reduces the binding and penetrating power of the Vam7p PX domain and leads to cytoplasmic redistribution of the EGFP-tagged protein. The affinities of the PX domain for PtdIns(3)P and other lipids reveal a remarkable synergy within the multivalent complex that stably anchors Vam7p at the vacuolar membrane.

Increased mobility in the membrane targeting PX domain induced by phosphatidylinositol 3-phosphate

Phosphoinositides (PIs) are concentrated in specific subcellular membranes in order to recruit and regulate cytosolic proteins responsible for vesicular trafficking, cytoskeletal rearrangement, and eukaryotic cell growth, differentiation, and survival. Phox homology (PX) domains are found in proteins that are integral players in endocytic pathways. For example, Vam7p is targeted by its PX domain to phosphatidylinositol 3-phosphate [PtdIns(3)P] in the yeast vacuole, where it interacts with other SNARE proteins and GTPases of the vesicular membrane fusion machinery. Although several PX structures have been solved, the role of dynamics in their interactions with membrane lipids is unclear. Here, we present the first detailed characterization of the backbone dynamics of a PX domain, that of Vam7p, in the presence and absence of its ligand. The structure appears to tumble more rapidly in solution upon binding PtdIns(3)P, revealing a conformational change that includes adjustments in the flexible membrane insertion loop (MIL). The flexibilities of the MIL and domain termini are pronounced in both states, while the alpha1 and alpha2 helices are rigid. Dynamic effects are spread across the binding pocket, with PtdIns(3)P inducing altered mobility of different residues on multiple timescales, including a shift in the MIL to slower timescale motions. The bound state is more dynamic overall, particularly in the beta-sheet lobe, which packs against the ligand's 3-phosphate. Thus, the induced dynamic and structural effects are transduced from the buried heart of the binding pocket in the helical lobe through the beta-sheet lobe to the exposed surface of the bilayer-inserted protein.

Targeting of the FYVE domain to endosomal membranes is regulated by a histidine switch

Specific recognition of phosphatidylinositol 3-phosphate [PtdIns3P] by the FYVE domain targets cytosolic proteins to endosomal membranes during key signaling and trafficking events within eukaryotic cells. Here, we show that this membrane targeting is regulated by the acidic cellular environment. Lowering the cytosolic pH enhances PtdIns3P affinity of the FYVE domain, reinforcing the anchoring of early endosome antigen 1 (EEA1) to endosomal membranes. Reversibly, increasing the pH disrupts phosphoinositide binding and leads to cytoplasmic redistribution of EEA1. pH dependency is due to a pair of conserved His residues, the successive protonation of which is required for PtdIns3P head group recognition as revealed by NMR. Substitution of the His residues abolishes PtdIns3P binding by the FYVE domain in vitro and in vivo. Another PtdIns3P-binding module, the PX domain of Vam7 and p40phox is shown to be pH-independent. This provides the fundamental functional distinction between the two phosphoinositide-recognizing domains. The presented mode of FYVE regulation establishes the unique function of FYVE proteins as low pH sensors of PtdIns3P and reveals the critical role of the histidine switch in targeting of these proteins to endosomal membranes.

Multivalent mechanism of membrane insertion by the FYVE domain

Targeting of a wide variety of proteins to membranes involves specific recognition of phospholipid head groups and insertion into lipid bilayers. For example, proteins that contain FYVE domains are recruited to endosomes through interaction with phosphatidylinositol 3-phosphate (PtdIns(3)P). However, the structural mechanism of membrane docking and insertion by this domain remains unclear. Here, the depth and angle of micelle insertion and the lipid binding properties of the FYVE domain of early endosome antigen 1 are estimated by NMR spectroscopy. Spin label probes incorporated into micelles identify a hydrophobic protuberance that inserts into the micelle core and is surrounded by interfacially active polar residues. A novel proxyl PtdIns(3)P derivative is developed to map the position of the phosphoinositide acyl chains, which are found to align with the membrane insertion element. Dual engagement of the FYVE domain with PtdIns(3)P and dodecylphosphocholine micelles yields a 6-fold enhancement of affinity. The additional interaction of phosphatidylserine with a conserved basic site of the protein further amplifies the micelle binding affinity and dramatically alters the angle of insertion. Thus, the FYVE domain is targeted to endosomes through the synergistic action of stereospecific PtdIns(3)P head group ligation, hydrophobic insertion and electrostatic interactions with acidic phospholipids.

Signaling with phosphoinositides: better than binary

When the outside of cell is stimulated,the inside generates a flurry of signals. Phosphates are sprinkled over lipids and proteins,where they are recognized within diverse signaling pathways. The kinases that congregate beneath the cell surface to provide the phosphate tags that mediate signaling have become major targets of new wave of drug design. Phosphoinositide signaling presents a particularly intriguing network whose many mysteries are now being unlocked. Research into protein domains that specifically recognize phosphoinositides have established the ENTH, FYVE,Phox,and pleckstrin homology domains s four cornerstones of phosphoinositide signaling.

Phox domain interaction with PtdIns(3)P targets the Vam7 t-SNARE to vacuole membranes

Specific recognition of phosphoinositides is crucial for protein sorting and membrane trafficking. Protein transport to the yeast vacuole depends on the Vam7 t-SNARE and its phox homology (PX) domain. Here, we show that the PX domain of Vam7 targets to vacuoles in vivo in a manner dependent on phosphatidylinositol 3-phosphate generation. A novel phosphatidylinositol-3-phosphate-binding motif and an exposed loop that interacts with the lipid bilayer are identified by nuclear magnetic resonance spectroscopy. Conservation of key structural and binding site residues across the diverse PX family indicates a shared fold and phosphoinositide recognition function.

Evaluation of immunoassays for semiquantitative detection of cocaine and metabolites or heroin and metabolites in extracts of sweat patches

Two types of immunoassays, radioimmunoassay (RIA) and microplate enzyme immunoassay (EIA), were compared for their ability to detect and quantitate cocaine and metabolites or heroin and metabolites in extracts of sweat patches. Experiments used sweat patches that had been fortified with cocaine, benzoylecgonine (BE), and ecgonine methyl ester (EME) or 6-acetylmorphine (6-AM), heroin, and morphine. Assays were first evaluated for sensitivity in detection of the analyte(s) known to be excreted in sweat (cocaine >> BE and EME; 6-AM > heroin > morphine). The cocaine metabolite RIA had cross-reactivity for cocaine > BE > EME, and the cocaine metabolite EIA had cross-reactivity for BE > cocaine >> EME. The RIA, having greater sensitivity for COC, was studied further. Optimal linearity was 4 to 200 ng/patch, and quantitation within these limits at 4, 75, and 150 ng/patch had intrarun %CVs within 7.8% and percent targets within 15% and inter-run %CVs within 13.5% and % targets within 13%. The opiate RIA had cross-reactivities for morphine >> 6-AM and heroin. The opiate EIA had cross-reactivities for 6-AM and heroin of 42 and 28% relative to morphine, respectively. The EIA, having greater sensitivity for 6-AM and heroin, was studied further. The limits of detection ranged from 1.7 to 24.7 ng/patch, and the lower limits of quantitation ranged from 7.3 ng/patch to beyond the linear range. The assay, however, had consistently good precision at 4 and 5 ng/patch, and optimal linearity was established from 4 to 100 ng/patch. With controls at 5, 25, and 90 ng/patch, both intrarun and inter-run precision were acceptable. Quantitation was accurate at 5 and 25 ng/patch, but the 90 ng/patch controls were consistently < 70% of target. Because our studies focused on the assays that had greater sensitivity for the analytes excreted in sweat, we did not fully evaluate the cocaine metabolite EIA or the RIA opiate screen and therefore cannot make any comment on the usefulness of these assays for detecting analytes in extracts of sweat patches beyond predicting that they will have less sensitivity. Both the cocaine metabolite RIA and opiate EIA had the ability to detect analytes known to be extracted from sweat patches.

Detection of methadone, LAAM, and their metabolites by methadone immunoassays

l-Alpha-acetylmethadol (LAAM) was recently approved as a substitute for methadone. LAAM, methadone, and their common metabolite, methadol, are extensively N-demethylated. The structural similarities of LAAM and its metabolites to methadone suggest that they may cross-react in methadone immunoassays. To test this hypothesis, drug-free urine was fortified with LAAM, norLAAM, dinorLAAM, methadol, normethadol, dinormethadol, methadone, 2-ethylidene-1,5-dimethyl-3,3-diphenylpyrrolidine (EDDP), or 2-ethyl-5-methyl-3,3-diphenylpyrroline (EMDP) at 12 concentrations (0.03 to 100 microg/mL). Samples were analyzed using two enzyme immunoassays (Behring Diagnostics, EIA-b; Diagnostic Reagents, EIA-d); a fluorescent polarization immunoassay (Abbott, FPIA); two enzyme-linked immunosorbant immunoassays (Diagnostix, ELISA-d; STC Technologies, ELISA-s); a kinetic microparticles in solution immunoassay (Roche Diagnostic Systems, KIMS); and a radioimmunoassay (Diagnostic Products, RIA). LAAM had high cross-reactivity with ELISA-d (318.3%), RIA (249.5%), EIA-d (100.8%), KIMS (91.1%), and ELISA-s (75.3%). Methadol also displayed relatively high cross-reactivity as follows: EIA-d (97.8%), KIMS (85.4%), ELISA-d (70.3%), and FPIA (37.7%). Successive N-demethylations of LAAM and methadol were associated with loss of cross-reactivity. The methadone metabolites EDDP and EMDP showed little cross-reactivity. These findings suggest that LAAM use could result in positive immunoassay test results when using many of the commercially available methadone immunoassay kits and that confirmation of LAAM and its metabolites should be considered.

Correlation of saliva codeine concentrations with plasma concentrations after oral codeine administration

A clinical study was designed to determine if there was a predictable relationship between saliva and plasma codeine concentrations. Drug-free volunteers (n = 17) were administered a 30-mg dose of liquid codeine phosphate. Plasma and saliva specimens were collected at various times for 24 h after administration. Plasma and saliva were analyzed for codeine and morphine by positive-ion chemical ionization gas chromatography-mass spectrometry. The plasma codeine concentrations peaked between 30 min and 2 h after administration and ranged from 19 to 74 ng/mL with a mean of 46 ng/mL. Despite decontamination procedures, elevated saliva codeine concentrations were detected at the early collection times because of contamination of the oral cavity from the liquid codeine. Codeine concentrations in the 15 min specimens ranged from 690 ng/mL to over 15,000 ng/mL. After the initial 2-h period, the mean codeine saliva concentrations declined at a rate similar to that observed in the plasma, but remained 3 to 4 times greater than the plasma concentrations. During the elimination phase, half-life estimates for codeine in plasma and saliva were found to be equivalent, 2.6 and 2.9 h, respectively. However, the area under the curve (AUC) estimate for codeine in saliva was 13 times greater than the plasma AUC. Contamination of the saliva resulted in elevated saliva/plasma (S/P) concentration ratios for the first 1 to 2 h after drug administration. Consequently, S/P ratios in specimens collected in the first 15 to 30 min ranged from 75 to 2580. However, after the absorption phase, a significant correlation between saliva and plasma concentrations was observed (r = 0.809, p < 0.05) and mean S/P ratios remained constant (mean = 3.7). Although small changes in saliva pH were predicted to produce profound changes in the S/P ratios for codeine, this was not observed in the current study. Therefore, saliva codeine concentrations could be used to estimate plasma concentrations through the use of the S/P ratio once the oral contamination has been eliminated. However, these estimates should be made cautiously. One must ensure that oral contamination is not a factor. Also, as with blood-drug concentrations, considerable intersubject variability was observed.

Quantitative measure of genetic differences in susceptibility to noise-induced hearing loss in two strains of mice

The CBA/CaJ (CB) and C57BL/6J (B6) inbred strains of mice were exposed for 1 h to noise intensities between 98 and 119 dB SPL. Previous studies indicated that the B6 mice exhibited permanent threshold shifts (PTS) after 1h exposure to 110 dB, whereas the CB mice did not exhibit any PTS. These differences in susceptibility to noise-induced hearing loss (NIHL) appear to be due to a gene for age-related hearing loss (AHL). The current study was designed to determine dose-response curves for NIHL over the ranges of intensities of noise that would characterize the B6 and CB inbred strains of mice. Because of the considerable differences in sensitivity to NIHL, the noise exposures for the two strains overlapped only at 110 and 113 dB. Nevertheless, the two strains exhibited two different dose-response curves, offset and with different slopes. We postulate that the B6 strain of mice exhibits a more linear increase for PTS from 98-113 dB, consistent with incremental effects on some metabolic physiological mechanism(s); the abrupt transition in NIHL between 113 and 116 dB for the CB mice is consistent with an ototraumatic structural injury.

A comparison of ONTRAK TESTCUP, abuscreen ONTRAK, abuscreen ONLINE, and GC/MS urinalysis test results

This study was designed to compare results obtained from two separate on-site drug testing kits (ONTRAK TESTCUP and Abuscreen ONTRAK) with those obtained from laboratory based immunoassay and GC/MS. Abuscreen ONLINE immunoassay was used to select 250 negative samples and 100 presumptive-positive samples each for cocaine/metabolites, opiates and cannabinoids. Presumptive-positive samples were selected if the immunoassay response was > or = 300 ng/mL for cocaine/metabolites (BZE), > or = 300 ng/mL for opiates or > or = 50 ng/mL for cannabinoids (THC-COOH). GC/MS was used to confirm that each selected sample contained > or = 150 ng/mL BZE, > or = 300 ng/mL morphine/codeine or > or = ng/mL THC-COOH. TESTCUP results had a 100% agreement with GC/MS and a > 99% agreement with ONLINE when testing negative samples. The agreement between TESTCUP and ONLINE results for samples containing opiates was 100%. Results of testing samples containing BZE with TESTCUP demonstrated a 98% agreement with both GC/MS and ONLINE. Both discrepant samples contained BZE at concentrations < or = 300 ng/mL. The least agreement between TESTCUP and ONLINE results was found when testing samples containing THC-COOH. The agreement with ONLINE and GC/MS was 92% and all discrepant samples had GC/MS determined THC-COOH concentrations less than 50 ng/mL. A 100% agreement was obtained between expected and recorded TESTCUP results for QC samples fortified to contained BZE, morphine or THC-COOH at concentrations within 120% of the screening cutoffs. ONTRAK had a 100% agreement with both GC/MS and ONLINE when testing negative samples and samples that contained opiates. ONTRAK had a 91% agreement with GC/MS and ONLINE for testing of samples that contained BZE. The least agreement between ONTRAK and ONLINE results was found when testing samples that contained THC-COOH. The agreement was 89%, however, all discrepant samples contained GC/MS concentrations of THC-COOH less that the 50 ng/mL cutoff. With ONTRAK, a 100% agreement was obtained between expected and recorded results QC samples that contained morphine or THC-COOH and a 97.7% agreement was obtained between expected and recorded results on QC samples that contained BZE.