Subnuclear Distribution of the S‐100 Protein Specific Binding Sites in Rat Brain

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Abstract
Fractionation of isolated rat brain nuclei previously reacted with 125I-labeled S-100 showed that most of the specifically bound radioactivity associated with the nuclear membranes and the nucleoli. Labeling of nucleoli, which indicates the entrance of 125I-labeled S-100 into the nucleus, was observed at 37.degree. C, but not at 0-4.degree. C. When tested separately for 125I-labeled S-100 specific binding, both the nuclear membranes and the nucleoli bound 125I-labeled S-100 in a biphasic manner, the binding displaying a high affinity and a low affinity component, as observed with intact nuclei. The binding to nuclear membranes was largely irreversible, while that to nucleoli was fully reversible after any association time.