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

The transport of secretory proteins from membrane-bound polysomes to extracellular space is a key activity in a variety of cells ; nevertheless, it is still incompletely understood. Much of the available information has come from a limited number of mammalian tissues, especially pancreas (1, 2, 3, 4) . The zymogen cells of silk moth galea are unusually favorable for studies of secretory protein transport. These cells are highly specialized for largescale secretion of a single protein, the zymogen of cocoonase (5) . After its synthesis, zymogen is quantitatively sequestered into a single, large storage 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 ; moreover, by the use of actinomycin D, synthesis of proteins other than zymogen may be virtually abolished (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.