Comparative Studies on the Characteristic Properties of Two Forms of Brain Actin Separable by Isoelectric Focussing

Actin isolated from 10‐day embryonic chick brain produced a single band indistinguishable from that of muscle actin when electrophoresed in sodium dodecyl sulfate‐polyacrylamide gel. Isoelectric focussing in polyacrylamide gel showed that the isolated protein was composed of two components–the β and γ forms of actin previously detected in other mammalian nonmuscle cells and tissues–in a molar ratio of 1.1/1.0. The same ratio was observed in a sonicate of 10‐day embryonic chick brain and in various actin‐containing fractions at each step of an actin purification procedure which involved gel filtration chromatography and polymerization‐depolymerization of the protein. Additionally, the two forms of actin were found to co‐precipitate with muscle myosin and to bind to a DNase I‐agarose affinity column in this ratio. In contrast, a third isoelectrically distinct protein with the same electrophoretic mobility as actin was found in the whole brain sonicate, but not in any of the actin‐containing fractions examined. When cellular protein in neuronal and non‐neuronal cell populations isolated from 10‐day embryonic chick brain was analyzed, it was found that β and β actins were also present in each class of cells in the molar ratio of 1.1/1.0. However, this ratio decreased slightly during development of the chick brain. We conclude from this study that the β and γ forms of brain actin are very similar in several characteristic properties of actin, and that it is unlikely that brain cells utilize different forms of the protein for different types of cell motility.