CALCIUM-INDUCED AGGREGATION OF NEUROENDOCRINE PROTEIN 7B2 IN-VITRO AND ITS MODULATION BY ATP

To study the behavior of the neuroendocrine polypeptide 7B2 in the pre sence of calcium, various fragments of this molecule were produced in Escherichia coli as fusion proteins to glutathione S-transferase (GST) . Addition of millimolar concentrations of Ca2+ to purified preparatio ns of hybrid molecules carrying the N-terminal segment of 7B2 induced precipitation in a manner dependent on protein and cation concentratio ns. This precipitation occurred at pH 7.5 but not at pH 5.2. It was au gmented by 4 and 8 mM ATP, and reduced by 12 and 24 mM ATP. ADP had a similar but weaker effect. Calcium failed to cause precipitation of GS T alone or of GST fused to the C-terminal peptide 7B2(156-186). Howeve r, when the latter protein was mixed with a GST protein carrying a sho rt fragment of the N-terminal region of 7B2, both proteins were precip itated by calcium. Except for the pH dependence, the behavior of 7B2 f usion proteins in the presence of calcium and adenosine nucleotides ar e reminiscent of those exhibited by chromogranins and secretogranins, which, like 7B2, are acidic proteins found in the secretory granules o f a variety of neuroendocrine cells. As suggested for other granins, t his property may underlie the segregation of 7B2 fragments into secret ory granules.