Following a labelled cargo

Cultured secretory cells receive a short pulse of labelled amino acid followed by unlabelled amino acid. For one cargo protein, the fraction of total recovered label in rough endoplasmic reticulum (ER), Golgi, vesicles and medium is measured. At 0, 10, 20 and 40 minutes the fractions are respectively: ER 80, 40, 10, 0%; Golgi 20, 45, 25, 0%; vesicles 0, 15, 45, 20%; medium 0, 0, 20, 80%. Total recovered label is constant across the untreated time course; its absolute value is not compared between treatments. In a separate treatment B, the 40-minute distribution is ER 5%, Golgi 75%, vesicles 15%, medium 5%. B does not change synthesis during the pulse. Assume label remains attached to the cargo throughout.