Phalloidin-gold complexes: a new tool for ultrastructural localization of F-actin.

We used a phalloidin-gold complex to study the distribution of F-actin in the myxamoebae and macroplasmodia of the slime mold Physarum polycephalum. After incubation of Lowicryl- or Quetol-embedded specimens with this complex, significantly different labeling intensities were found over the various cytoplasmic and nuclear regions of the cells. The nucleoplasm was the most heavily labeled cell compartment, followed in decreasing order of labeling intensity by the cytoplasm, the nucleolus, and the chromocenters. The labeling observed over the latter area did not appear significantly different from that of the background. Sections incubated in the phalloidin-gold complex to which an excess of F-actin was added showed no significant labeling over any of the above-mentioned cell regions. Other control experiments included incubation of the sections with a phalloidin solution followed by the phalloidin-gold complex, PEG-stabilized colloidal gold, and a bovine serum albumin-gold complex. There was no or very little labeling of the preparations.