Assessment of membrane protein expression and stability using a split green fluorescent protein reporter

Membrane proteins are challenging targets for structural biologists. Finding optimal candidates for such studies requires extensive and laborious screening of protein expression and/or stability in detergent. The use of green fluorescent protein (GFP) as a reporter has enormously facilitated these studies; however, its 238 residues can potentially alter the intrinsic properties of the target (e.g., expression or stability). With the aim of minimizing undesired effects of full-length GFP, here we describe the utility of a split GFP reporter during precrystallization studies of membrane proteins. GFP fluorescence appeared by complementation of the first 15 residues of GFP (GFP(11)) (fused to the C terminus of a membrane protein target) with the remaining nonfluorescent GFP (GFP(1-10)). The signal obtained after sequential expression of SteT (l-serine/l-threonine exchanger of Bacillus subtilis) fused to GFP(11) followed by GFP(1-10) specifically measured the protein fraction inserted into the Escherichia coli cytoplasmic membrane, thereby discarding protein aggregates confined as inclusion bodies. Furthermore, in vitro complementation of purified SteT-GFP(11) with purified GFP(1-10) was exploited to rapidly assess the stability of wild-type and G294V mutant versions of SteT-GFP(11) following detergent solubilization and purification. This method can be applied in a medium- to high-throughput manner with multiple samples.