Subcellular sorting of isoactins: Selective association of γ actin with skeletal muscle mitochondria

We describe two subpopulations of actin antibodies isolated by affinity chromatography from a polyclonal antibody to chicken gizzard actin. One subpopulation recognizes γ actins from smooth muscle and nonmuscle cells, but does not recognize α actin from skeletal muscle. The other subpopulation recognizes determinants that are common to a actin from skeletal muscle and the two γ actin isotypes. Neither antibody recognizes cytoplasmic β actin. Both antibodies recognize only actins or molecules with determinants that are also present in actins. By immunofluorescence we found that the anti-γ actin colocalizes with mitochondria in fibers of mouse diaphragm, and that it does not bind detectably to the I bands of sarcomeres. The antibody that recognizes both α and γ actins stains I bands intensely, as expected. We interpret these observations as preliminary evidence for selective association of γ actin with skeletal muscle mitochondria and, more broadly, as evidence for subcellular sorting of isoactins.