Characterization of Fluorescently Labeled Protein with Electrospray Ionization-MS and Fluorescence Spectroscopy: How...

Area of Science:

• Biochemistry
• Protein Chemistry
• Fluorescence Spectroscopy

Background:

• Solvent-exposed lysine residues are common targets for protein labeling with extrinsic probes.
• Neutrophil gelatinase-associated lipocalin (NGAL) is a serum protein and a diagnostic marker for acute kidney injury.
• Fluorescent labeling is widely used for protein detection and characterization.

Purpose of the Study:

• To investigate the fluorescent properties and localization of AlexaFluor488 (AF488) and Cy3 labels on NGAL.
• To understand the cause of fluorescence quenching observed with AF488 labeling.
• To challenge the assumption of random probe distribution in protein labeling.

Main Methods:

• Protein labeling with AF488 and Cy3 using standard protocols.
• Quantification of labeling efficiency using UV-vis absorption and electrospray ionization mass spectrometry (ESI-MS).
• Identification of label attachment sites using liquid chromatography-tandem mass spectrometry (LC-MS/MS) and trypsin digestion.

Main Results:

• High labeling efficiency (>95%) was achieved for AF488, but fluorescence intensity was <10% of expected.
• AF488 predominantly attached to lysine residues K125, K126, or K135 within the NGAL calyx, likely quenched by nearby aromatic residues.
• Cy3 labeling retained full fluorescence, with predominant attachment to lysine K31 on the protein surface.

Conclusions:

• The microenvironment and inherent properties of fluorescent labels influence their attachment sites and fluorescence.
• Specific lysine residues in NGAL, particularly those in the calyx, can quench AF488 fluorescence.
• Protein labeling is not always a random process; site-selectivity can occur, impacting probe performance.