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Department of Cell Biology, University of Virginia Health Sciences Center, Charlottesville, Virginia 22908
Previous studies have suggested that salivary
amylase and proline-rich protein are sorted differently
when expressed in AtT-20 cells (Castle, A.M., L.E.
Stahl, and J.D. Castle. 1992. J. Biol. Chem. 267:13093-
13100; Colomer, V., K. Lal, T.C. Hoops, and M.J. Rindler. 1994. EMBO (Eur. Mol. Biol. Organ.) J. 13:3711- 3719). We now show that both exocrine proteins behave
similarly and enter the regulated secretory pathway as
judged by immunolocalization and secretagogue-
dependent stimulation of secretion. Analysis of stimulated secretion of newly synthesized proline-rich protein, amylase, and endogenous hormones indicates that
the exogenous proteins enter the granule pool with
about the same efficiency as the endogenous hormones.
However, in contrast to the endogenous hormones,
proline-rich protein and amylase are progressively removed from the granule pool during the process of
granule maturation such that only small portions remain in mature granules where they colocalize with the
stored hormones. The exogenous proteins that are not
stored are recovered from the incubation medium and are presumed to have undergone constitutive-like secretion. These results point to a level of sorting for regulated secretion after entry of proteins into forming
granules and indicate that retention is essential for efficient storage. Consequently, the critical role of putative
sorting receptors for regulated secretion may be in retention rather than in granule entry.
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