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4. A scientist purified a protein from animal cells and made fluorescent antibodies to it. When she isolated pieces of the plasma membrane from another batch of animal cells and added the fluorescent antibodies, she could see fluorescence on only one side of the membrane using a fluorescence microscope. She then took some fresh pieces of plasma membrane and suspended them in a salt solution before adding the fluorescent antibodies. The plasma membrane did not become fluorescent. The protein was most likely: