The specific interaction of S-100 protein with synaptosomal particulate fractions. Site-site interactions among S-100 binding sites

Chlorpromazine inhibits the calcium-mediated effects of S-100 protein(s) on assembled brain microtubule proteins, but not those on microtubule protein assembly

We have examined the S-100-chlorpromazine interplay at the level of brain microtubule proteins in vitro. The results indicate that in the presence of 0.12 M KCl and 10 microM free Ca2+ the inhibitory effect of S-100 on microtubule assembly is additive to that of chlorpromazine, but S-100 fails to potentiate the disassembling effect of 0.1 mM Ca2+ if added to assembled microtubule proteins after chlorpromazine and Ca2+, probably because of inhibition of S-100 by the phenothiazine. Chlorpromazine does not compete with S-100 for binding to purified tubulin.

Effect of S-100 protein on assembly of brain microtubule proteins in vitro

S-100 protein inhibits the assembly of brain microtubule proteins in vitro in the presence of 10 microM free Ca2+. The S-100 effect is generally greater on the rate than on the extent of assembly, and even greater as the microtubule protein concentration decreases and the time of preincubation between S-100 and microtubule proteins before GTP addition increases, at a given S-100/tubulin dimer molar ratio. The S-100 effect is greatly enhanced in the presence of physiological concentrations of K+ and is completely reversed by EGTA.

Biochemical and physicochemical properties of the solubilized S-100 protein binding activity of synaptosomal particulate fractions

The 125I-labeled S-100 specific binding to a Triton X-100 (TX-100) extract of synaptosomal particulate fractions (SYN) was investigated. The results indicate that (a) S-100 binding to the TX-100 extract is partially irreversible after a critical association time at 37 degrees C, while it is fully reversible after any association time at 4 degrees C; (b) trypsin and phospholipase C partially reverse the S-100 binding, while phospholipase D enhances the interaction to some extent, in a dose-dependent way; (c) EDTA and high concentrations of NaCl or KCl are more efficient as inhibitors of the S-100 binding to the TX-100 extract than as 125I-labeled S-100 dissociating agents, in analogy with previous observations with SYN; and (d) two main populations of solubilized S-100 binding sites can be evidenced by gel filtration and sucrose gradient centrifugation when low amounts of the TX-100 extract are processed and/or low S-100 concentrations are used, while two additional molecular species are separated when greater amounts of either factors are tested. These results suggest the possibility that S-100 may be involved in the regulation of some membrane activities.

Spectral studies on the calcium binding properties of bovine brain S-100b protein

The effect of Ca2+ binding on the circular dichroism (CD) and 270-MHz proton nuclear magnetic resonance (NMR) spectra of brain-specific S-100b calcium binding protein has been examined at two pH values, 8.5 and 7.5. At pH 8.5, S-100b protein binds two Ca2+ per monomer with Kd values of 6 x 10(-5) and 2 x 10(-4) M, whereas at pH 7.5, the protein binds only one Ca2+ per monomer with a Kd of 2 x 10(-4) M. The presence of K+ inhibits the binding of Ca2+ to the higher affinity site at pH 8.5, and the affinity for calcium is lowered to Kd = 8.5 x 10(-4) M. Mg2+ has no effect on protein conformation. In the absence of Ca2+, S-100b undergoes a conformational change when the protein is titrated from pH 8.6 to 6.0. Addition of Ca2+ perturbed the environment of tyrosine and phenylalanine residues as measured by ultraviolet difference spectroscopy and 1H NMR. CD melt experiments and far-ultraviolet CD studies at alkaline pH and NMR experiments suggest that the protein is more stable in the presence of Ca2+. The single tyrosine residue in the protein ionizes only after the protein is denatured by exposure to high pH.

Immunocytochemical localization of S-100 protein binding sites in synaptosomal fractions and subfractions

1. Immunocytochemical evidence is presented of the ultrastructural localization of binding sites for S-100 protein in synaptosomal fractions and subfractions. Synaptosomes or isolated synaptosomal subfractions were first incubated with S-100, then centrifuged to remove unbound S-100, and finally fixed before treatment with anti-S-100 antiserum, using the unlabeled antibody peroxidase-antiperoxidase (PAP) method. 2. When intact synaptosomes were used, the immunoreaction product was localized to the postsynaptic density including the postsynaptic membrane. In some reactive synaptosomes, the presynaptic membrane was labeled as well, in the region of synaptic contact. No reaction deposit was found in the synaptic cleft or on intrasynaptosomal structures. Divalent cations (Ca2+ and Mg2+) were essential for S-100 to interact with synaptosomes. Of the synaptosomal subfractions tested, i.e., synaptic vesicles and intraterminal mitochondria, only synaptic vesicles showed immunoreactivity when treated with S-100 and anti-S-100 antiserum as described above. 3. The S-100 immunoreactivity in synaptic structures documented in this report parallels the distribution of the high-affinity binding sites for radiolabeled S-100 in synaptosomal fractions and subfractions.

Identification and characterization of sulfhydryl-containing proteolytic fragments involved in the Ca2+-induced conformational change of beef brain S-100

The Ca2+-dependent conformational alteration of the brain-specific S-100 protein was studied by reacting the protein with N-ethyl[2,3-14C]maleimide in the absence and presence of Ca2+ and under denaturing conditions. Peptic hydrolysates of the 14C-labeled protein were analyzed and fractionated by high-performance liquid chromatography. Labeled peptide fractions were characterized by high-voltage electrophoresis and TLC. A clear distinction could be made between two classes of sulfhydryl-containing fragments: (a) neutral, hydrophobic, and (b) acidic. Ca2+ markedly favored 14C incorporation into the former components, whereas the latter were readily available only under denaturing conditions.

Heterogeneity of the S-100 protein specific binding sites in synaptosomal particulate fractions and subfractions

The specific interaction of S-100 protein with synaptosomal particulate fractions (SYN) was further investigated with special reference to the number of binding components and their localization in synaptosomal subfractions. Binding studies were conducted on SYN from various CNS regions, on synaptosomal subfractions from the cerebral cortex, and on cerebral cortex SYN under various conditions. The results suggest that S-100 binds to two populations of membrane sites: high -affinity sites, which seem to be confined to neuronal membranes (synaptosomal plasma membranes and synaptic vesicles), and low-affinity sites, which are also detected in other membranes. The data are consistent with the view that the biphasic profile of S-100 binding to SYN does not result from heterogeneity of the S-100 molecule, and that the Ca2+ conformation of the protein is as important as the proper conformation of the binding site for full expression of high-affinity binding.

The specific interaction of S-100 protein with synaptosomal particulate fractions. Modulation of binding by fractional occupancy of sites and by the physical state of membranes

The nonlinearity of single components of the Scatchard plot of S-100 binding to synaptosomal particulate fractions (SYN) and the observation that dilution of the 125I-labeled S-100 site complex results in a greater extent of dissociation of the tracer in the presence than in the absence of an excess of unlabeled S-100 suggest that sites change their binding behavior depending on fractional occupancy. To study this aspect of the interaction in more detail, 125I-labeled S-100 binding experiments were conducted in the presence of, or after preincubation of SYN with various concentrations of, unlabeled S-100. The results indicate that: (a) S-100 synaptosomal sites do change their binding behavior depending on fractional occupancy; and (b) the nonrapid equilibrium between bound S-100 and the medium, which has been referred to as the formation of a tight complex between S-100 and its binding sites, is related to the activation of high-affinity sites. However, no univocal interpretation of these data in terms of binding model can be offered at present, as the binding models currently employed in the analysis of ligand-site interactions can each account for only part of the results described in this report. In any case, data obtained by studying 125I-labeled S-100 binding to untreated SYN at 2 degrees C and to prefixed SYN at 37 degrees C indicate that the physical state of membranes influences both the extent of the interaction and the binding behavior of the sites.

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.

The specific interaction of S-100 protein with synaptosomal particulate fractions. Evidence for the formation of a tight complex between S-100 and its binding sites

The dissociation of the 125I-labelled S-100 specifically bound to synaptosomal particulate fractions (SYN) has been studied under a variety of conditions after different association times. The results indicate that after a critical association time of about 20 min at 37 degrees C, the bound protein becomes progressively less accessible to the dissociating agents of conditions employed. These findings support the view that the partial irreversibility of the 125I-labelled S-100 binding to SYN could be due to the formation of a tight complex between the protein and its synaptosomal sites. These data are discussed mainly in relation to the particulate-bound fraction of native S-100.