Organelles are not uniformly distributed throughout the cytoplasm but have preferred locations that vary between tissues and during development. To investigate organelle targeting to cytoplasmic domains we have taken advantage of the mouse pituitary cell line, AtT-20, which, when induced to extend long processes, accumulates dense core secretory granules at the tips of the processes. During mitosis, these secretory granules accumulate along the plane of division. Protein synthesis is not mandatory for such redistribution of secretory granules. To explore the specificity of the redistribution we have used transfected AtT-20 cells that express the immunoglobulin kappa light chain. While the endogenous hormone ACTH is found in secretory granules, the kappa chain is a marker for organelles involved in constitutive secretion. By immunofluorescence, kappa also accumulates at the tips of growing processes, and along the midline of dividing cells, suggesting that the redistribution of vesicles is not specific for dense-core secretory granules. Since there is evidence for selective organelle transport along processes in neuronal cells, the rat pheochromocytoma cell PC-12 was transfected with DNA encoding markers for regulated and constitutive secretory vesicles. Again regulated and constitutive vesicles co-distribute, even in cells grown in the presence of nerve growth factor. We suggest that at least in the cells studied here, cytoskeletal elements normally carry exocytotic organelles to the surface; when the cytoskeletal elements coalesce in an extending process, exocytotic organelles of both the constitutive and regulated pathway are transported nonselectively to the tips of the cytoskeletal elements where they accumulate.

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