Labeling effects on the isoelectric point of green fluorescent protein

We studied the effects of fluorescent labeling on the isoelectric points (p l values) of proteins using capillary isoelectric focusing with laser-induc ed fluorescence detection (cIEF-LIF). Specifically, we labeled green fluore scent protein (GFP) from the jellyfish Aequorea victoria with the fluorogen ic dye 3-(2-furoyl)quinoline-2-carboxaldehyde (FQ). cIEF-LIF was used to mo nitor the native fluorescence of GFP and showed pi changes in GFP's FQ-labe led products. Multiple labeling of GFP with FQ produced a series of product s with pi values shifted towards a low pH. We verified cIEF-LIF results wit h traditional slab gel IEF. Our cIEF-LIF technique can routinely detect 10( -11) M of FQ-labeled protein, whereas traditional slab gel IEF with silver stain detection gives detection limits of 10(-7) M in the same samples. (C) 1999 Published by Elsevier Science B.V. All rights reserved.