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THE PACKAGING OF A SECRETORY PROTEIN : Kinetics of Cocoonase Zymogen Transport into a Storage Vacuole

The Journal of Cell Biology
The Rockefeller University Press
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THE PACKAGING OF A SECRETORY PROTEIN Kinetics of Cocoonase Zymogen Transport into a Storage Vacuole FOTIS C. KAFATOS and VASSILI KIORTSIS . From the Biological Laboratories, Harvard University, Cambridge, Massachusetts 02138, and the Zoological Laboratory, University of Athens, Athens, Greece INTRODUCTION The transport of secretory proteins from mem- brane-bound polysomes to extracellular space is a key activity in a variety of cells ; nevertheless, it is still incompletely understood. Much of the avail- able information has come from a limited number of mammalian tissues, especially pancreas (1, 2, 3, 4) . The zymogen cells of silk moth galea are unusu- ally favorable for studies of secretory protein trans- port. These cells are highly specialized for large- scale secretion of a single protein, the zymogen of cocoonase (5) . After its synthesis, zymogen is quantitatively sequestered into a single, large stor- age vacuole in a pure form (6), facilitating the monitoring of transport by radioautography (7) . The cells can be maintained for at least 2 days in organ culture at a variety of developmental stages . In vivo, during a 7 day differentiated phase (8), zymogen synthesis progressively accelerates, while synthesis of nonsecretory proteins decreases ; more- over, by the use of actinomycin D, synthesis of pro- teins other than zymogen may be virtually abol- ished (7) . Finally, these invertebrate cells afford a unique opportunity for comparative studies of secretion . The present report concerns the kinetics of zymogen transport into the storage vacuole of cultured galea cells at various developmental stages. MATERIALS AND METHODS Galeae were dissected and cultured in serum-free Grace's (9) medium (Grand Island Biological Co., Berkeley, Calif.) as previously described (7) . Each galea was cut transversely into three segments ; thus, six segments were available per animal . All tissues were given a simultaneous 15 m

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