The role of actin in the secretory cycle.

Binding of filamentous actin to isolated insulin storage granules has been demonstrated in an in vitro system; this binding is enhanced by 2 mM ATP, but is diminished in the presence of 1 mM CaCl2. It is unaffected by glucose or by cyclic AMP. Estimation of the actin content of isolated rat islets by DNAase inhibition indicates that G-actin constitutes 1 to 2% of the protein content of the islets, that 50 to 70% of the actin content of the islets is present as the depolymerized globular from (G-actin), and that the proportion of F-actin found may be increased in islets incubated in the presence of 20mM glucose. Thus both the proportions of G- and F-actin, and the binding of granules to F-actin may be important in the regulation of rates of insulin secretion.