Reorganization of alpha-fodrin induced by stimulation in secretory cells

Spectrin is an ubiquitous protein composed of heterodimers with alpha and beta subunits. It was first described in erythrocyte cell membranes (see ref. 1 for review) and subsequently in brain, intestinal brush borders, kidney, liver and adrenals. Brain spectrin (fodrin) alpha-subunit, responsible for actin binding, has a relative molecular mass (Mr) of 240,000, whereas the beta-subunit, involved in membrane attachment, has an Mr of 235,000 (refs 1, 3, 9-13). The membrane of secretory granules from adrenal chromaffin cells membrane of secretory granules from adrenal chromaffin cells increases the viscosity of F-actin solution, and spectrin-like protein is associated with storage granule and plasma membranes. Here, we report the localization of fodrin in secretory cells using monospecific antibodies against the alpha-subunit of fodrin using indirect immunofluorescence. We find that the alpha-subunit forms an intensely stained continuous ring in the subplasmalemmal region of resting chromaffin cells. On stimulation of the cell with nicotine, high potassium or ionophores in the presence of calcium, fodrin forms patches. This aggregation is inhibited by trifluoperazine, hence the entrance of calcium into cells following cell depolarization seems to be the calmodulin-dependent stimulus initiating patch formation.