A stable human-cell system overexpressing cystic fibrosis transmembrane conductance regulator recombinant protein at the cell surface.

Recent human clinical trials results demonstrated successful treatment for certain genetic forms of cystic fibrosis (CF). To extend treatment opportunities to those afflicted with other genetic forms of CF disease, structural and biophysical characterization of CF transmembrane conductance regulator (CFTR) is urgently needed. In this study, CFTR was modified with various tags, including a His10 purification tag, the SUMOstar (SUMO*) domain, an extracellular FLAG epitope, and an enhanced green fluorescent protein (EGFP), each alone or in various combinations. Expressed in HEK293 cells, recombinant CFTR proteins underwent complex glycosylation, compartmentalized with the plasma membrane, and exhibited regulated chloride-channel activity with only modest alterations in channel conductance and gating kinetics. Surface CFTR expression level was enhanced by the presence of SUMO* on the N-terminus. Quantitative mass-spectrometric analysis indicated approximately 10% of the total recombinant CFTR (SUMO*-CFTR(FLAG)-EGFP) localized to the plasma membrane. Trial purification using dodecylmaltoside for membrane protein extraction reproducibly recovered 178 ± 56 µg SUMO*-CFTR(FLAG)-EGFP per billion cells at 80% purity. Fluorescence size-exclusion chromatography indicated purified CFTR was monodisperse. These findings demonstrate a stable mammalian cell expression system capable of producing human CFTR of sufficient quality and quantity to augment future CF drug discovery efforts, including biophysical and structural studies.

Site-preferential dissociation of peptides with active chemical modification for improving fragment ion detection.

Multiple reaction monitoring tandem mass spectrometry becomes an important strategy for measuring protein targets in complex biomatrixes. Active chemical modification of peptides like phenylthiocarbamoylation has unique potential for improving the measurement. This potential is enabled by active participation of a modifying group in site-preferential dissociation of modified peptides, which produces certain fragment ions at very high yields and in a sequence-independent manner. In this work, a novel combination of energy-resolved mass spectrometry with substituent effect investigation is used to analyze important factors that control the specificity of the site-preferential dissociation of phenylthiocarbamoyl peptides. On the basis of the linear correlation between collision energy and the Hammett constant as well as computational studies, it is found that the initial enhanced capture of a mobile proton and the subsequent, site-directed intramolecular proton transfer are important to the high yields (approximately 70-90%) for producing two types of fragment ions of phenylthiocarbamoyl peptides: the modified b(1) ion and the complementary y(n-1) ion. This understanding will help the design of new modification reagents. When integrated with the throughput and the signal-enhancing potential of peptide modification, active chemical modification of peptides will significantly advance mass spectrometry-based, targeted proteome analysis.

Averagine-scaling analysis and fragment ion mass defect labeling in peptide mass spectrometry.

A method termed as the averagine-scaling analysis (ASA) is proposed for predictive design and selection of chemical reagents for modifying peptides, as well as for facile mass spectral analysis of peptide fragment ions with increased mass defects. The ASA method scales mass spectral data using the mass of the hypothetical averagine residue as reference. The scaling analysis is used in conjunction with a strategy of fragment ion mass defect labeling (FIMDL) for effectively using the broad, unoccupied mass zones in the low m/ z region of mass spectra. The FIMDL approach involves the solution modification of peptide termini with chemical reagents of large mass defects and the gas-phase generation of peptide terminal fragment ions that carry the FIMDL groups. The scaling analysis reveals that iodine has the highest FIMDL efficiency among halogens. Iodine-containing reagents, 4-iodophenylisocyanate and 4-iodophenylisothiocyanate, are used to label primary amines on peptides to demonstrate the scaling analysis. The ASA method successfully distinguishes peptide fragment ions with and without an FIMDL group and specifically and efficiently reduces the data complexity of peptide tandem mass spectra. The combination of ASA with FIMDL extends the instrument suitability for the mass defect analysis from mass spectrometers of ultrahigh mass resolution and accuracy to those of medium ones. This combination is expected to have a profound impact on peptide tandem mass spectrometry.