Abstract
By use of dark-field light microscopy, secretory granules isolated from the anglerfish endocrine pancreas were observed to attach to and release from microtubules assembled in vitro from brain homogenates. Secretory granules only bound to microtubules assembled in the presence of microtubule-associated proteins (MAPs) and not to microtubules assembled from purified tubulin. The addition of a MAP fraction to purified tubulin restored secretory granule binding. The secretory granules were released from MAP-containing microtubules by the addition of Mg-ATP but not by other nucleotides. The number of secretory granules bound to MAP-containing microtubules was increased in the presence of cyclic AMP. In addition to the associations of secretory granules with microtubules, MAP-containing microtubules also associated with each other. These laterally associated microtubules were dispersed by the addition of Mg-ATP. Electron micrographs confirmed that the associations between MAP-containing microtubules and secretory granules as well as the associations of microtubules with one another were mediated by the high molecular weight MAPs known to project from the surface of in-vitro-assembled microtubules.
Highlights
By use of dark-field light microscopy, secretory granules isolated from the anglerfish endocrine pancreas were observed to attach to and release from microtubules assembled in vitro from brain homogenates
The effect of 3',5'-cyclic AMP on the associations was examined because (a) Sloboda et al [51] reported that cyclic AMP stimulated the phosphorylation of MAP-2, (b) our results indicated that one of the high molecular weight microtubule-associated proteins (MAPS) is responsible for linking secretory granules to microtubules, and (c) cyclic AMP has been found to be important for the secretion of insulin by beta cells [6, 8, 60]
A number of morphological studies have indicated that such linkages occur in situ [1, 20, 34, 40, 42, 43, 52, 59] and that the connections appear to be mediated by filamentous structures similar to the high molecular weight MAPS associated with the walls of microtubules assembled in vitro from brain homogenates [13, 28, 37]
Summary
By use of dark-field light microscopy, secretory granules isolated from the anglerfish endocrine pancreas were observed to attach to and release from microtubules assembled in vitro from brain homogenates. More direct evidence for the involvement of microtubules in beta granule movements comes from morphological studies of beta granules in monolayer cell culture. In these cells, the membranes of the beta granules were frequently closely associated with or directly linked to microtubules [34, 40]. Similar associations between microtubules and organelle membranes have been found in other cells [1, 20, 40, 42, 43, 52] While these studies suggest that various organelles are linked to microtubules in vivo, little is known about the nature of the interactions. Sherline et al [49] studied the associations between microtu-
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