THE ALPHA-CHAIN OF THE NASCENT POLYPEPTIDE-ASSOCIATED COMPLEX FUNCTIONS AS A TRANSCRIPTIONAL COACTIVATOR

We report the characterization of clone 1.9.2, a gene expressed in min eralizing osteoblasts. Remarkably, clone 1.9.2 is the murine homolog o f the alpha chain of the nascent polypeptide-associated complex (alpha -NAC), Based on sequence similarities between alpha-NAC/1.9.2 and tran scriptional regulatory proteins and the fact that the heterodimerizati on partner of alpha-NAC was identified as the transcription factor BTF 3b (B. Wiedmann, H. Sakai, T. A. Davis, and M. Wiedmann, Nature 370:43 4-440, 1994), we investigated a putative role for alpha-NAC/1.9.2 in t ranscriptional control, The alpha-NAC/1.9.2 protein potentiated by 10- fold the activity of the chimeric activator GAL4/VP-16 in vivo. The po tentiation was shown to be mediated at the level of gene transcription , because alpha-NAC/1.9.2 increased GAL4/VP-16-mediated mRNA synthesis without affecting the half-life of the GAL4/VP-16 fusion protein, Mor eover, the interaction of alpha-NAC/1.9.2 with a transcriptionally def ective mutant of GAL4/VP-16 was severely compromised, Specific protein -protein interactions between alpha-NAC/1.9.2 and GAL4/VP-16 were demo nstrated by gel retardation, affinity chromatography, and protein blot ting assays, while interactions with TATA box-binding protein (TBP) we re detected by immunoprecipitation, affinity chromatography, and prote in blotting assays, Based on these interactions that define the coacti vator class of proteins, we conclude that the alpha-NAC/1.9.2 gene pro duct functions as a transcriptional coactivator.