RAPIDLY LABELED AND SECRETED PROTEINS OF THE CHICK BRAIN

Abstract— The extracellular and cerebrospinal fluids (ECF) of the chick brain were found to contain a distinctive group of rapidly labeled proteins. Gel staining patterns suggest that most ECF protein bands correspond with components also found in either the homogenized whole brain cytoplasmic fraction or the blood serum. The valine-incorporation profiles of these three fractions, however, were entirely distinctive. Comparisons were carried out using a sensitive double-labeling method, in which ECF proteins from chicks labeled for 1 h with ³H]valine were comigrated on SDS-polyacrylamide gels with the cytoplasmic or serum proteins from a ¹⁴C-labeled animal. Analyses of the ³H- and ¹⁴C-labeling profiles from these gels showed that certain newly-synthesized proteins are heavily enriched in the ECF relative to the other two fractions. Most prominently, material with an apparent molecular weight of # 17,000 was found to incorporate nearly one-third of all the radioactivity appearing in the ECF proteins, but was not heavily labeled in either the cytoplasmic or serum fractions. The effects of a simple training experience on the pattern of chicks' brain protein synthesis were also examined.