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 mineralizing osteoblasts. Remarkably, clone 1.9.2 is the murine homolog of the alpha chain of the nascent polypeptide-associated complex (α-NAC). Based on sequence similarities between α-NAC/1.9.2 and transcriptional regulatory proteins and the fact that the heterodimerization partner of α-NAC was identified as the transcription factor BTF3b (B. Wiedmann, H. Sakai, T. A. Davis, and M. Wiedmann, Nature 370:434–440, 1994), we investigated a putative role for α-NAC/1.9.2 in transcriptional control. The α-NAC/1.9.2 protein potentiated by 10-fold the activity of the chimeric activator GAL4/VP-16 in vivo. The potentiation was shown to be mediated at the level of gene transcription, because α-NAC/1.9.2 increased GAL4/VP-16-mediated mRNA synthesis without affecting the half-life of the GAL4/VP-16 fusion protein. Moreover, the interaction of α-NAC/1.9.2 with a transcriptionally defective mutant of GAL4/VP-16 was severely compromised. Specific protein-protein interactions between α-NAC/1.9.2 and GAL4/VP-16 were demonstrated by gel retardation, affinity chromatography, and protein blotting assays, while interactions with TATA box-binding protein (TBP) were detected by immunoprecipitation, affinity chromatography, and protein blotting assays. Based on these interactions that define the coactivator class of proteins, we conclude that the α-NAC/1.9.2 gene product functions as a transcriptional coactivator.