Subnuclear distribution of the S-100 protein specific binding sites in rat brain

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