How do you store membrane proteins?
I work with membrane proteins and in my experience some proteins can be frozen and thawed without any problems, other proteins just don't like that. It's linked to the stability of the protein. If you really need to freeze the protein you may think about playing around with different detergents to see if you can improve the stability of the protein.

General rules:
- freezing membrane proteins at -80°C without glycerol
- avoiding protein freeze-thaw cycles, making aliquots before freezing
- keeping them at 4°C only for short term use
- after concentration always spinning the protein (10 min at 100k for example) to precipitate aggregated material. Aggregation causes aggregation, so you want to get rid of it.