Microheterogeneity of Brain Cytoplasmic and Synaptoplasmic Actins

Actin present in whole rat brain cytoplasm and in synaptosomes was purified by DNase I affinity chromatography. By use of two‐dimensional gels and one‐dimensional isoelectric focusing gels, brain actin was shown to be composed of two isomeric forms. By comparison with muscle actins, brain actins were identified as the β and γ isomers. Muscle type α actin is not present in brain. Synaptosomal protein with high affinity for DNase I is primarily composed of β and γ actin, however, two minor synaptosomal proteins, S1 and S2, with similar DNase I affinity were also isolated. S11 and S2 have the same apparent molecular weight as whole brain actin, are more acidic than the major actin forms and are distinct from a actin. Relative to β and γ actin, the content of S1 and S2 is 3‐fOld greater in synaptosomes when compared to similar non‐synaptosomal species. The results demonstrate heterogeneity of brain actins and compartmentalization of brain proteins with high affinity for DNase I at the synapse. It was also shown that tubulin has selective affinity for the DNase I‐actin complex.