Luminescence Resonance Energy Transfer to Study Conformational Changes in Membrane Proteins Expressed in Mammalian Cells

Summary

We describe here an improved Luminescence Resonance Energy Transfer (LRET) method where we introduce a protease cleavage site between the donor and acceptor fluorophore sites. This modification allows us to obtain specific LRET signals arising from membrane proteins of interest, allowing for the study of membrane proteins without protein purification.

Abstract

Luminescence Resonance Energy Transfer, or LRET, is a powerful technique used to measure distances between two sites in proteins within the distance range of 10-100 Å. By measuring the distances under various ligated conditions, conformational changes of the protein can be easily assessed. With LRET, a lanthanide, most often chelated terbium, is used as the donor fluorophore, affording advantages such as a longer donor-only emission lifetime, the flexibility to use multiple acceptor fluorophores, and the opportunity to detect sensitized acceptor emission as an easy way to measure energy transfer without the risk of also detecting donor-only signal. Here, we describe a method to use LRET on membrane proteins expressed and assayed on the surface of intact mammalian cells. We introduce a protease cleavage site between the LRET fluorophore pair. After obtaining the original LRET signal, cleavage at that site removes the specific LRET signal from the protein of interest allowing us to quantitatively subtract the background signal that remains after cleavage. This method allows for more physiologically relevant measurements to be made without the need for purification of protein.

Introduction

Luminescence Resonance Energy Transfer (LRET) is a derivative of the well-known Fluorescence Resonance Energy Transfer (FRET) technique¹. Similar to FRET, LRET can be used to measure distances and distance changes between donor and acceptor fluorophores attached to specific sites on the protein of interest within the range of 10-100 Å^1-3. The principles of LRET are also similar to FRET in that resonance energy transfer occurs between two proximal fluorophores when the emission spectrum of the donor fluorophore overlaps with the absorption spectrum of the acceptor fluorophore. The efficiency of this transfer is related to the distance between the ....

Protocol

1. Create the Construct Containing the Protein of Interest

Clone the gene expressing the protein of interest into a suitable vector. Use vectors such as the pcDNA series or pRK5 as they are well suited for expression in mammalian systems like HEK293 and CHO cells.

2. Select the Sites on the Protein to be Tagged with the Fluorophores

1. Choose labeling sites that are able to reflect possible conformational changes within the protein. If possible, use a crystal structure of the protein or of a homologous protein to help make this determination. If there is no crystal structure available, use online software ....

Results

A successful LRET measurement with a lanthanide donor should have a donor only lifetime in the millisecond time range. The lifetime of sensitized LRET emission for the protein labeled with both the donor and the acceptor will be notably shorter, with a lifetime in the microsecond range after background subtraction Figure 3. Protease cleavage results in an increase in lifetime that should become stable over time (i.e. no longer changing), showing that the protease cleavage is complete Fig.......

Discussion

LRET is a powerful technique that allows scientists to measure distances between domains within a single protein as well as between subunits in a multimeric protein. As such, LRET is well-suited to examining the conformational changes and dynamics of proteins or other macromolecules. The above protocol should allow the properly equipped lab to easily test their hypotheses; however, there are many common sources of error that may plague the new investigator. If little or no LRET signal is seen, first check the wavelength .......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
LanthaScreen Thiol Reactive Tb Chelate	Life Technologies	PV3579
Acceptor fluorophore-Fluorescein-5-Maleimide	Life Technologies	F-150	Choice of acceptor depends on the specific experiment
Acceptor fluorophore-Alexa Fluor 488 C5 Maleimide	Life Technologies	A-10254	Choice of acceptor depends on the specific experiment
Acceptor fluorophore-Alexa Fluor 555 C2 Maleimide	Life Technologies	A-20346	Choice of acceptor depends on the specific experiment
Acceptor fluorophore-Alexa Fluor 594 C5 Maleimide	Life Technologies	A-10256	Choice of acceptor depends on the specific experiment
Acceptor fluorophore-Alexa Fluor 680 C2 Maleimide	Life Technologies	A-20344	Choice of acceptor depends on the specific experiment
QuantaMaster 3-SS	Photon Technology International		Spectrofluorometer should have pulsed excitation with the ability to measure lifetimes in the millisecond range
FluoreScan 2.0	Photon Technology International		Data Acquisition Software used in manuscript. Software provided with fluorescence instrument
Origin 8.6	OriginLab		Origin is the data analysis software used in protocol. Can use other similar data analysis software
Quartz cuvette	Starna Cells, Inc	3-Q-10
Stir bar	Bel-Art Products	F37119-0007	Used in cuvette to keep cells in suspension. Can use any stir bar that fits the cuvette