Preparation of highly 3H-labeled S-100 protein under nondenaturing conditions

The S-100: a protein family in search of a function

The S-100 is a group of low molecular weight (10-12 kD) calcium-binding proteins highly conserved among vertebrates. It is present in different tissues as dimers of homologous or different subunits (alpha, beta). In the nervous system, the S-100 exists as a mixture composed of beta beta and alpha beta dimers with the monomer beta represented more often. Its intracellular localisation is mainly restricted to the glial cytoplasmic compartment with a small fraction bound to membranes. In this compartment the S-100 acts as a potent inhibitor of phosphorylation on several substrates including the synaptosomal C-Kinase and Tau, a microtubule-associated protein. The S-100 in particular conditions, after binding with specific membrane sites (Kd = 0.2 microM; Bmax = 4.5 nM), is able to modify the activity of adenylate cyclase, probably via G-proteins. In addition, the Ca2+ homeostasis is also modulated by S-100 via an increase of specific membrane conductance and/or Ca2+ release from intracellular stores. "In vitro" and "in vivo" experiments showed that lower (nM) concentrations of extracellular S-100 beta act on glial and neuronal cells as a growth-differentiating factor. On the other hand, higher concentrations of the protein induce apoptosis of some cells such as the sympathetic-like PC12 line. Finally, data obtained from physiological (development, ageing) or pathological (dementia associated with Down's syndrome, Alzheimer's disease) conditions showed that a relationship could be established between the S-100 levels and some aspects of the statii.

Quantitative assay of S-100 protein in mouse brain cortex synaptosomes

1. Data on the presence of S-100 protein in synaptic endings are revised, and evidence is given in favor of its localization inside mouse brain cortex synaptosomes and on the surface of their external membrane. 2. For identification of the S-100-specific polypeptide, proteins of external synaptosomal membranes were iodinated with lactoperoxidase fixed on cyanogen bromide (CNBr)-Sepharose, and after synaptosome lysis S-100-positive material was isolated by means of affinity chromatography antibodies to S-100 protein (a-S-100)-Sepharose. The molecular weight of the polypeptide obtained corresponded to that of S-100 subunits (10 kD), and iodine incorporation pointed to its localization on the surface of synaptosomal membranes. 3. With the help of antibodies labeled with horseradish peroxidase (a-S-100-HP) or 125I (a-S-100-125I), which do not penetrate into noninjured synaptosomes, the amount of S-100 protein on synaptosomal membranes was found to be 18.5 ng/mg total protein (as assayed with a-S-100-HP) or 95.33 ng/mg (as assayed with a-S-100-125I). 4. At the same time, the total S-100 protein content in synaptosomes measured by means of radioimmune analysis after their complete lysis turned out to be 284 +/- 0.84 ng/mg, i.e., a part of S-100 seemed to be inside synaptosomes. 5. Cosedimentation of water-soluble S-100 protein with the synaptosomal fraction during isolation was insignificant. Prefixation with glutaraldehyde or paraformaldehyde decreased the amount of material reacting with antibodies, possibly due to steric effects or denaturation of active centers. This could have influenced the earlier attempts to detect S-100 protein in synapses. Treatment of nonfixed synaptosomes with a conjugate of a-S-100 with colloidal gold made it possible to detect S-100-positive material on pre- and postsynaptic membranes, which confirms the biochemical data.

The S-100 protein causes an increase of intracellular calcium and death of PC12 cells

The S-100 protein-PC12 cell interaction has been studied as a model system of the possible physiological role played by S-100 protein in the nervous system. The data reported demonstrate that S-100 exerts a cytotoxic action which eventually leads to PC12 cell death, regardless of the cell cycle phase. The effect is specific for the S-100 isoforms, which are made up of two identical subunits and is abolished by a monoclonal antibody directed against the same isoforms. Other isoforms and/or calcium-binding proteins, such as troponin or calmodulin, do not induce the same effects. The action of S-100 on cell viability is not detectable in other cell lines of different embryological origin, such as 3T3, L1210, GH3. S-100 causes a rapid and considerable increase (two- to three-fold) of intracellular Ca2+ concentration in PC12 cells accompanied by cytostatic and cytotoxic action. It is postulated that this action also occurs in vivo, as part of the physiological action of this protein.

Characterization of Ca2+-binding proteins from Ehrlich ascites tumor cell cytoplasm and their binding to membranes

A set of proteins in the 33-37 kDa range have been isolated from the cytoplasm of the Ehrlich ascites tumor cell. The proteins are characterized by their Ca2+-dependent binding to cell membranes. This property has been used for isolation of the proteins by Ca2+-dependent affinity binding to inside-out vesicles of the human red cell membrane. The proteins display Ca2+-binding properties as shown by gel-filtration studies. The Ca2+-dependent binding of the 33 and 34 kDa proteins to red cell membranes was studied after labelling of the proteins with tritium by reductive methylation. The average number of Ca2+ bound per protein molecule was 4.8 with a Kd of 3.4.10(-4) M Ca2+. The proteins are distinct from most other Ca2+-binding proteins of comparable molecular weights by not incorporating phosphate.

Highly efficient and simple methods for the preparation of peroxidase and active peroxidase-antibody conjugates for enzyme immunoassays

The periodate-mediated conjugation of horseradish peroxidase to antibody is one of the most popular methods to prepare conjugates for enzyme immunoassays of antigens or corresponding antibodies. A very simple method to obtain peroxidase, which is both about five times cheaper than the rather expensive commercial preparations and has a significant higher activity, is reported. Moreover, the conjugation method was critically investigated and considerably simplified. Conjugates thus obtained are about three times more active than the best obtained with the original method.

Isolation of homologous and heterologous complexes between catalytic and regulatory components of adenylate cyclase by forskolin-Sepharose

Homologous and heterologous complexes between catalytic and GTP-binding components can be isolated by means of immobilized succinyldeacetylforskolin (forskolin-Sepharose). A heterologous complex is formed by reconstitution of forskolin-Sepharose bound catalytic function from rabbit myocardial membranes with the homogenous [3H]methyl-GTP-binding protein from duck erythrocyte membranes. Analysis of the reconstituted complex by sodium dodecyl sulfate polyacrylamide gelelectrophoresis reveals that only the Mr 42 000 component of the GTP-binding protein's Mr 42 000/Mr 35 000 heterodimer contributes to the formation of active adenylate cyclase.

Chemistry and biology of the S-100 protein

The S-100 protein is specific for the nervous system and, being present in all vertebrates, shows a high degree of stability of structure during evolution. In adult animals it is primarily localized to glial elements, although there is some evidence that a small proportion may be present in neuronal nuclei or their plasma membranes. During development it is synthesized rapidly at a relatively late period of differentiation of the nervous system. In glioma cell cultures there is a control mechanism that seems to involve some kind of signal at the external surface of the plasma membrane, possibly specific cell-cell contact, to stimulate S-100 synthesis. All of these biological properties of S-100 suggest that it is connected with some specific essential function that is common to the nervous system of all vertebrates. Several chemical properties of S-100 provide clues to this function. It is an unusually acidic and soluble protein and, in the absence of Ca2+, has no detectable hydrophobic regions accessible to solvent. It is capable of specifically binding Ca2+, a process that causes S-100 to undergo a conformational change that exposes a hydrophobic region to the solvent and stimulates binding of S-100 to membranes. The conformational change and the membrane-binding properties are reversible when Ca2+ is removed and are antagonized by monovalent cations such as K+ and Na+. These chemical properties suggest that S-100 may, as part of its function in the nervous system, be bound to some hydrophobic site, possibly a membrane, and that the extent of this binding is regulated by concentrations of Ca2+, K+ and Na+. If this is true, then it is important, as the next step in working out its function, to discover the exact site where S-100 binds in the nervous system.