Distribution of anionic sites on intracellular organelles: A study by labeling frozen thin-section with cationized ferritin.

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Abstract
The distribution of anionic sites on intracellular organelles were studied by labeling frozen thin-sections with the multivalent ligand, cationized ferritin, as a visual probe. Frozen thin-sections obtained from fixed rat cardiac muscle with 0.5% glutaraldehyde were incubated with cationized ferritin (50 .mu.g/l ml phosphate buffered saline, pH 7.4) for 30 min at room temperature. The ligands almost continuously labeled the surface of the sarcolemma and the junctional membrane forming the intermediate junction and desmosome, and the luminal surface of the T-tubules. In mitochondria, the ligand binding occurred to the outer surface of the outer mitochondrial membranes and to the matrix side of the inner mitochondrial membranes. High affinity for the ligand was seen on the cytoplasmic side of the gap junction. In addition, the sarcoplasmic reticulum and thick filaments showed moderately high affinity for cationized ferritin. The implications of our observations in the regulation or maintenance of cellular organization are discussed.