Ultrastructural studies on actin-like filament in mung bean mitochondria and its potential functional significance.

This document reports that actin-like filaments, about 5 nm in width, were found in mung bean mitochondria after negative staining of mitoplast. The 5 nm filaments, composed of globular proteins, were visualized beneath the intramembranous particles by freeze-fracture electron microscope. Freeze-fracture immuno-labeling against anti-actin antibody suggested that actin-like filaments attached to the protoplasmic surface of mitochondrial inner membrane. These filaments were also found in the matrix and underneath of outer membrane. The presence of actin-like protein in mitochondria was also substantiated by immunolocalization in situ. Interestingly, some 5 nm filamentous bundles were often found between mitochondrion and its "bud". It is likely that these images represent an undiscovered novel plant mitochondrial budding process which may involve some 5 nm filamentous structure in separating nascent mitochondrion to its mother mitochondrion. Treatment of cultured tobacco cells with F-actin depolymerization reagent, Latrunculin B (Lat B) might affect the shapes of mitochondria. Taken together, this study suggests that some 5 nm filaments, probably F-actin, may be localized in mitochondria and play a role in mitochondria] propagation and mitochondrial shaping.