Multitasking C2H2 zinc fingers link Zac DNA binding to coordinated regulation of p300-histone acetyltransferase activity.

Zac is a zinc finger (ZF) protein which potently induces apoptosis and cell cycle arrest and prevents tumor formation in nude mice (44, 52). Expression of Lot1, the rat orthologue of Zac, is lost during spontaneous transformation of ovary surface epithelial cells in vitro (1), and the human orthologue ZAC/LOT1/PLAGL1, which is widely expressed in normal tissues, is frequently down-regulated in a methylation-sensitive manner in various tumors (4, 5). Evidence that Zac expression during embryogenesis in mesenchymal and neural stem/progenitor cells is tightly regulated in a spatiotemporal fashion and evidence that Zac is upregulated following seizures and transient focal cerebral ischemia in mice suggest that the protein may have additional roles, e.g., in neural differentiation and plasticity (2, 15, 48-50). Indeed, Zac was recently recloned in a subtractive hybridization screen for genes involved in neuronal cell fate specification (31). Moreover, Zac expression can be influenced by hormonal and epigenetic signals during regeneration, differentiation, and age-related degenerative processes, although its exact role in these conditions requires further studies (9, 13, 46, 57). In adults, Zac is highly expressed in most steroid-responsive tissues (35, 36, 48, 52). Interestingly, Zac potently coactivates or corepresses the hormone-dependent activity of nuclear receptors, including that of androgen, estrogen, glucocorticoid, and thyroid hormone receptors (20); all of these receptors are key regulators of cell growth, differentiation, homeostasis, and development and act in a cell-specific manner. Furthermore, recent studies disclosed the importance of Zac's imprinting status in the etiology of transient neonatal diabetes mellitus, an uncommon form of childhood diabetes (NCBI entry OMIM *601410 [http://www.ncbi.nlm.nih.gov/entrez/dispomim.cgi?id=601410]) (28, 51). Our earlier studies revealed that Zac can act as a transcription factor through its monomeric or dimeric binding either to a GC-rich palindromic DNA element or to GC-rich direct and reverse repeat elements, respectively (6, 18, 52). The two closely related members of the Zac family Plag1 and Plagl2 share at their N termini virtually identical DNA-binding domains consisting of canonical zinc fingers of the C2H2 type (52). In line with this observation, Zac and Plag target genes identified so far (18, 54, 58) contain closely related GC-rich sequences in their promoter regions. Zac family members show, however, marked differences in their expression patterns in several tissues, indicating different biological roles. Moreover, proteins of the Zac family strongly diverge in their C termini, which could determine specific interactions in transcriptional regulation. In fact, only Zac has been reisolated in a yeast screen for mammalian proteins that bind to the C-terminal activation domain of the nuclear receptor coactivator SRC-1 (GRIP1) (20). Moreover, Zac binds to the C-terminal domain of the homologous coactivators p300 and CREB-binding protein (CBP) and strongly coactivates or corepresses nuclear receptors and p53 in a context-dependent manner (19, 20, 39). The transcriptional coactivator proteins p300/CBP exert key roles in cellular differentiation, growth control, and homeostasis (16). In response to diverse physiological cues, these proteins coordinate and integrate multiple signal-dependent events at the transcriptional level by virtue of their histone acetyltransferase (HAT) activity. This allows them to influence chromatin activity by modulating nucleosomal histones, modify transcription factors, and influence DNA recognition, protein-protein interactions, and protein stability (25). The HAT activity of these coactivators is markedly increased upon their interaction with specific transcription factors (24, 43), although the underlying mechanisms of this switch in transcriptional activity are poorly understood. Whereas the classic recruitment model proposes that coactivation simply reflects the capacity of the activator to recruit the coactivator, we demonstrate here that HAT activity is controlled by the coordinated binding of Zac to p300. Furthermore, our experiments reveal a new function of C2H2 zinc fingers in the regulation of HAT activity, suggesting a dynamic role of DNA-binding proteins in the enzymatic control of transcription.