There is no way for a membrane transporter to demonstrate its transport function without the presence of a membrane-bound compartment. The traditional method of demonstrating that the purified protein retains its activity is to reconstitute it into a lipid bilayer. For an ion channel, the black lipid membrane electrophysiology technique can be used to measure the conductance of the reconstituted channel. For other types of transporters, a proteoliposome-based technique can be used.

Some transporters have functions other than transport that can be measured in detergent solution. For example, many member of the ABC superfamily, such as P-glycoprotein/MDR, will hydrolyze ATP in detergent solution, and the rate of ATP hydrolysis can be either enhanced or inhibited by adding transport substrates that bind to the protein. If the transporter has known ligands, you may be able to measure the affinity of the ligands with the protein in detergent solution.

You may partially mimic the phospholipid bilayer environment by forming nanodiscs for membrane protein including transport functionalized. Both solubility and affinity can be obtained under theoretically native conditions for this kind of proteome and activity tests produce reasonable results at in-vitro conditions....If I remind it correctly, Cubebiotech offers nanodisc reagents and publishes exemplified applications for suggested strategies...