Chromaffin cell cytoskeleton: its possible role in secretion.

Cytoskeleton proteins (actin, myosin, alpha-actinin, spectrin, tubulin, neurofilament subunits) and their regulatory proteins (calmodulin, gelsolin) have been isolated from adrenal chromaffin cells and characterized. Their physicochemical properties have been studied and their cell localizations have been revealed by biochemical, immunocytochemical, and ultrastructural techniques. alpha-Actinin and spectrin are components of chromaffin granule membranes and some of the cell actin copurifies with these secretory granules. Myosin is not detected in the granules, but is present mainly in the cytosol and close to the cell surface. Trifluoperazine, a calmodulin antagonists, blocks stimulation-induced hormone release from chromaffin cells at a step distal from Ca2+ entry. High affinity calmodulin-binding sites have also been found in chromaffin granule membranes and their calmodulin-binding proteins have been characterized. Furthermore, microinjection of calmodulin antibodies into chromaffin cells blocks hormone output in response to stimulation. In view of the above findings, the possible roles of contractile proteins and calmodulin in chromaffin cell functions are discussed.