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Polyribosomes, carrying nascent polypeptide chains, were prepared from whole brain, cortex, and hindbrain-medullary white matter of young adult rats. In a homologous cell-free system, a brain-specific protein (S100 protein) was identified in the mixture of polypeptides released from the polyribosomes during incubation for 1 hr at 37°. De novo synthesis of the S100 protein was achieved in a reconstituted cerebral cell-free system containing polysome-derived mRNA and 40S + 60S subunits. The radioactively labeled S100 protein synthesized in vitro was identified by precipitation with antibody to S100 after addition of purified S100 as a carrier, and migration of the solubilized precipitate on acrylamide gels in the presence of sodium dodecyl sulfate. In vitro synthesis of the S100 protein did not occur in analogous cell-free systems derived from hepatic tissue or in a heterologous system containing liver polyribosomes and cerebral enzymes.One of the active areas of research on mammalian protein biosynthesis has been the identification of products formed during cell-free incorporation of amino acids by polyribosomal preparations. The demonstration of active incorporation into globin chains by particles from reticulocytes was the first example of clear-cut evidence for an identifiable product formed in vitro (1-3). Some of the more recent and numerous examples include the demonstration of active incorporation into serum proteins and nonserum liver proteins by particulate preparations from hepatic tissue (4-11), into myosin by polyribosomes from embryonic-chick muscle (12) and myoglobin by polysomes from duck skeletal muscle (13), into heavy and light chains of antibodies by polysomes from rabbit lymph nodes (14, 15) and ovalbumin by chick-oviduct polysomes (16), and into B-lactoglobulin by particles from ewe mammary gland (17).Although cell-free systems dervied from brain are highly active in amino-acid incorporation (18-21), identification of products formed in vitro is difficult because of the complexity and heterogeneity of cerebral tissue. Nevertheless, Rubin and Stenzel (22) reported the in vitro synthesis of a soluble acidic protein (S100 protein) by a ribosomal system from rabbit cortical gray matter. Mahler and Brown (21) later reported that in a polysomal system from rat brain, about 33% of the counts released into the soluble portion during cell-free protein synthesis were found among several acidic proteins, but identification of a specific protein was not attempted in their studies. The isolation, purification, and characterization of two brain-specific low molecular weight proteins by Moore et al., as well as preparation of antisera to these proteins, has made possible a study of the in vitro synthesis of specific brain proteins by cerebral polyribosomes. The S100 protein (23) is primarily of glial origin (24-26), and represents 0.2% of the total soluble protein of brain. The other purified protein (14-3-2) is localized primarily in neurons (27). The function of these proteins is unknown, but t...