Chromaffin Granule Membrane‐F‐Actin Interactions and Spectrin‐Like Protein of Subcellular Organelles: A Possible Relationship

The membrane of chromaffin granule, the secretory vesicle of adrenal medullary cells storing catecholamines, enkephalins, and many other components, interacts with F‐actin. Using low shear falling ball viscometry to estimate actin binding to membranes, we demonstrated that mitochondrial and plasma membranes from chromaffin cells also provoked large increases in viscosity of F‐actin solutions. Mitochondrial membranes also had the capacity to cause complete gelation of F‐actin. In addition, vasopressin‐containing granules from neurohypophysial tissue were shown to bind F‐actin and to increase the viscosity of F‐actin solutions. Using an antibody directed against human erythrocyte spectrin, it was found that a spectrin‐like protein was associated with secretory granule membrane, mitochondrial membrane, and plasma membrane. The chromaffin granule membrane‐associated spectrin‐like protein faces the cytoplasmic side, is composed of two subunits (240 kD and 235kD), the α‐subunit (240 kD, pH i 5.5) being recognized by the antibody. Nonionic detergents such as Triton X‐100 or Nonidet P40 failed to release fully active spectrin‐like protein. In contrast, Kyro EOB, a different nonionic detergent, was found to release spectrin‐like protein while keeping intact F‐actin binding capacity, at least below 0.5% Kyro EOB concentration. Chromaffin cells in culture were stained with antispectrin antibody, showing the presence of spectrin‐like protein in the cell periphery close to the cell membrane but also in the cytoplasm. We conclude that in living cells the interaction of F‐actin with chromaffin granule membrane spectrin observed in vitro is important in controlling the potential function of secretory vesicles.