Molecular organization of rat prolactin granules: in vitro stability of intact and "membraneless" granules

Abstract

Studies carried out on a number of secretory cell systems suggest that the specific cytoplasmic granules in which the secretion products are stored before their release are complex organelles which can possess a distinct molecular organization. For instance, it has been reported that in some granules the segregated secretion products are organized into crystalline structures (1-3) or large intermolecular aggregates (4-8). It is likely that all phenomena of this type are favorable to the economy of the cell, in the sense that they reduce the energy required for storage of the secretion products. The prolactin (LTH) granules of the rat pituitary possess a number of morphological features which strongly suggest that the molecules(s) of their content might be arranged in a relatively stable structure. Thus, these granules are remarkably polymorphic in shape, and their membrane is usually separated from their content by a clear space. Furthermore, identifiable LTH granules devoid of their membrane are often seen in the pericapillary space, suggesting that upon discharge by exocytosis they are dissolved only slowly (9). However, no studies specifically concerned with the mechanisms of LTH storage have been reported so far. In order to obtain some information on this question, we have studied the behavior of isolated granule fractions incubated in vitro under a variety of carefully controlled experimental conditions.

Acquisition of Lubrol Insolubility, a Common Step for Growth Hormone and Prolactin in the Secretory Pathway of Neuroendocrine Cells

“…At longer times after synthesis, some of the intracellular 35 S-prolactin became insoluble in Lubrol (Fig. 1), a property of prolactin aggregates in the dense cores of secretory granules (2). Over 50% of the total 35 S-prolactin (intracellular plus extracellular) was insoluble 30 min after the end of the incubation with 35 S-amino acids in the experiments summarized in Fig.…”

“…Giannattasio et al (2) isolated intact prolactin-containing secretory granules from rat pituitary glands and then removed the granule membranes with detergent. When the nonionic detergent Lubrol was used, the granule membranes dissolved, but the prolactin contents remained insoluble, and the dense cores of the granules retained their morphology (2).…”

“…Giannattasio et al (2) isolated intact prolactin-containing secretory granules from rat pituitary glands and then removed the granule membranes with detergent. When the nonionic detergent Lubrol was used, the granule membranes dissolved, but the prolactin contents remained insoluble, and the dense cores of the granules retained their morphology (2). Dense cores of secretory granules with no surrounding membrane appear occasionally in the pericapillary space of the pituitary gland, indicating that the cores dissolve relatively slowly after exocytosis in animals (3,4).…”

Parotid Secretory Granules: Crossroads of Secretory Pathways and Protein Storage

“…Thus, the granule core appears to be loosely organized and does not contain aggregated secretory proteins. In contrast, the endocrine secretory granule cores can be highly organized, and some resist hypotonic conditions, suggesting that the contents are not osmotically active (Giannattasio et al, 1975). In pancreatic β-cells, the granule cores consist of crystallized insulin (Coore et al, 1969), while the cores of pituitary secretory granules can be isolated from purified secretory granules after membrane dissolution (Zanini et al, 1980).…”

Distinct molecular mechanisms for protein sorting within immature secretory granules of pancreatic beta-cells.

“…There is general agreement that insulin and many other regulated secretory proteins are packaged into an insoluble dense core (Giannattasio et al, 1975;Michael et al, 1987), which is a general feature of exocrine and neuroendocrine storage granules (Palade, 1975;Kelly, 1985). Two current models of targeting to storage granules (sorting for entry [Bauerfeind and Huttner, 1993] and sorting by retention [Kuliawat and Aryan, 1992]) differ in whether the condensation of regulated secretory proteins need or need not occur prior to protein entry into IGs.…”