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Abstract

Green fluorescent protein fused to human chromogranin B or neuropeptide y was expressed in PC12 cells and caused bright, punctate fluorescence. The fluorescent points colocalized with the endogenous secretory granule marker dopamine β−hydroxylase. Stimulation of live PC12 cells with elevated [K⁺], or of permeabilized PC12 cells with Ca²⁺, led to Ca²⁺−dependent loss of fluorescence from neurites. Ca²⁺ stimulated secretion of both fusion proteins equally well. In living cells, single fluorescent granules were imaged by evanescent−wave fluorescence microscopy. Granules were seen to migrate; to stop, as if trapped by plasmalemmal docking sites; and then to disappear abruptly, as if through exocytosis. Evidently, GFP fused to secreted peptides is a fluorescent marker for dense−core secretory granules and may be used for time−resolved microscopy of single granules