deep freez -80

If the protein was delivered as a solid, you will have to dissolve it in a buffer. Proteins should not be stored in a highly dilute state, so make a solution of at least 1 mg/ml. Divide it into many small aliquots (such as 20-50 µl) in separate tubes, flash freeze the tubes on dry ice, and store them at -80oC or in liquid nitrogen in very small tubes. Then you can avoid multiple freeze-thaws. However, many proteins do not lose activity as the result of being frozen and thawed as long as it is done properly (flash freeze, thaw quickly, store on ice during use.) It is helpful to include 20% glycerol in the storage buffer, which acts as a stabilizer and also makes it quicker to thaw the aliquot in your hand.

If the protein was delivered in solution, then do the same as above, skipping the dissolving step.

Another alternative is to make a buffered solution of the protein containing 50% glycerol. This can be stored at -20oC without it freezing. Then the needed amount of the solution can simply be withdrawn from a single tube containing the stock solution.

Thank you Dr Adam and Dr Yahia for your suggestion. 

Thank you Dr Shirley Dong