Bone-Specific Expression of the Alpha Chain of the Nascent Polypeptide-Associated Complex, a Coactivator Potentiating c-Jun-Mediated Transcription

Tissue-specific gene expression is regulated by a combination of sequence-specific DNA-binding transcriptional activators, general or basal transcription initiation factors, and associated cofactors. The sequence-specific transcription factors can be divided into several classes or families based on either the activation domain they possess or the protein motif they use for DNA binding. In addition to these site-specific proteins, accurate transcription initiation by RNA polymerase II requires at least six general transcription initiation factors: TFIIA, TFIIB, TFIID, TFIIE, TFIIF, and TFIIH (reviewed in references 16 and 29). While the general initiation factors are sufficient for basal-level transcription, enhancement of transcription by transcriptional activator proteins bound to enhancer elements requires the presence of additional mediator proteins, known as transcriptional coactivators (12, 16, 19, 38). Coactivators are defined functionally by their ability to selectively potentiate the stimulatory activity of specific subsets of enhancer binding transcriptional activators. Among the best-characterized coactivators are the TAFs (TATA box-binding protein [TBP]-associated factors) (12), which are subunits of TFIID. Purification and molecular cloning of several TAFs have confirmed that they provide protein interfaces to link the sequence-specific factors to the basal transcriptional machinery, and some exhibit specific enzymatic functions essential for activated gene transcription (11, 27, 28). Moreover, coactivators distinct from those associated with TBP have also been identified and cloned (reviewed in reference 19). We have recently demonstrated that the alpha chain of the nascent polypeptide-associated complex (α-NAC) protein, previously thought to be involved in some aspects of translational control (41), could translocate to the nucleus, where it was shown to function as a transcriptional coactivator in conjunction with the chimeric activator GAL4/VP-16 in vivo (43a). Specific interactions between α-NAC and TBP were detected. Examination of the expression pattern of α-NAC in adult tissues allowed us to identify and characterize a muscle-specific isoform of α-NAC, skNAC, also involved in the regulation of gene transcription (43). In this report, we have examined the expression pattern of α-NAC during embryogenesis and observed that it was selectively expressed in developing bone. In an attempt to identify endogenous transcriptional activators that could interact with α-NAC to regulate gene transcription in differentiating bone cells, we examined the possibility that AP-1 proteins, known modulators of bone development in vivo (17), might represent natural targets for α-NAC. The AP-1 proteins are formed through the heterodimerization of Fos family members and Jun family members through a structural motif called the leucine zipper (30). The heterodimer can then bind DNA at a consensus site termed the AP-1 site and act as a transcription factor to modulate the expression of AP-1-responsive genes (30). All of the Jun family members can also homodimerize to exert the same function (30). Interestingly, α-NAC was shown to potentiate the activity of the homodimeric c-Jun activator, while transcription mediated by the c-Fos/c-Jun heterodimer was unaffected. We have delineated the domain of the c-Jun protein that interacts with α-NAC and measured the influence of α-NAC on kinetic parameters of c-Jun binding to AP-1 sites. These observations have allowed us to propose a model for the α-NAC-mediated coactivation of c-Jun-dependent transcription. The observed restricted expression pattern of α-NAC during embryogenesis and the identification of its interaction with c-Jun suggest new perspectives in the study of the regulation of gene transcription during osteoblastic differentiation.