Pancreatic islet cells were cultured in a medium containing radio-labeled amino acids. Every five minutes, a sample of cells were removed from the culture, washed and fractionated. The radioactivity of each fraction was then measured. The results showed that in the first five minutes, most of the radioactivity was found in the rough ER. After 30 minutes, most of the radioactivity was found in the Golgi body. After 60 minutes, almost all of the radioactivity was found in the cell membrane. Which of the following is most likely the identity and function of the radio-labeled protein?
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