The 14-3-3 protein is a family of acidic proteins present exclusively in the brain and is believed to have a function in monoamine biosynthesis because of its ability to activate tyrosine hydroxylase and tryptophan hydroxylase in the presence of Ca2+/calmodulin-dependent protein kinase type II. In this study, we resolved bovine brain 14-3-3 protein into seven polypeptide components by means of reversed-phase chromatography and determined the amino acid sequence of one of these components (eta chain) by cloning its cDNA from a bovine cerebellum cDNA library. The eta-chain mRNA is 1.8 kilobases long and encodes a polypeptide of 246 amino acids and Mr 28,221. Computer-assisted analysis of the sequence indicates that the eta chain exhibits no internal sequence repeats, nor does it have significant sequence similarity to other proteins with known amino acid sequence. However, the eta chain appears to consist of two structural regions that are distinguishable in their clearly different charge characteristics: the almost neutral amino-terminal region and the strongly acidic carboxyl-terminal region. The structural features of the eta chain and the domain organization of tyrosine and tryptophan hydroxylases suggest that the 14-3-3 protein binds to the regulatory domain of the phosphorylated hydroxylases through its acidic carboxyl-terminal region and activates the hydroxylases by inducing an active conformation.