Specific Binding Sites for S-100 Protein in Isolated Brain Nuclei

Isolated brain nuclei possess binding sites for S-100 protein. The interaction of S-100 with these sites is specific and time-, temperature-, and Ca+ -dependent. The profile of the (125)I-labelled S-100 binding inhibition is biphasic, displaying a high-affinity component and a low-affinity component. The S-100 binding to brain nuclei is largely irreversible, probably owing to the formation of a tight complex between the protein and its nuclear binding sites. The S-100 binding to brain nuclei is in most aspects similar to that to synaptosomal membranes. Several lines of evidence indicate, however, that the S-100 binding to nuclei is not due to contamination of these structures with plasma membranes. Isolated liver nuclei do not possess the high-affinity component of S-100 binding.