We and others recently isolated a human p53 homologue (p40͞p51͞p63͞p73L) and localized the gene to the distal long arm of chromosome 3. Here we sought to examine the role of p40͞p73L, two variants lacking the N-terminal transactivation domain, in cancer.

Although transcription and pre-mRNA processing are colocalized in eukaryotic nuclei, molecules linking these processes have not previously been described. We have identified four novel rat proteins by their ability to interact with the repetitive C-terminal domain (CTD) of RNA polymerase II in a yeast two-hybrid assay. A yeast homolog of one of the rat proteins has also been shown to interact with the CTD. These CTD-binding proteins are all similar to the SR (serine/arginine-rich) family of proteins that have been shown to be involved in constitutive and regulated splicing. In addition to alternating Ser-Arg domains, these proteins each contain discrete N-terminal or C-terminal CTD-binding domains. We have identified SR-related proteins in a complex that can be immunoprecipitated from nuclear extracts with antibodies directed against RNA polymerase II. In addition, in vitro splicing is inhibited either by an antibody directed against the CTD or by wild-type but not mutant CTD peptides. Thus, these results suggest that the CTD and a set of CTDbinding proteins may act to physically and functionally link transcription and pre-mRNA processing.The C-terminal domain of the largest subunit of RNA polymerase II (CTD) consists of tandem repeats of the consensus sequence Tyr-Ser-Pro-Thr-Ser-Pro-Ser (1, 2). Deletion studies demonstrated that the ClD is essential for cell growth (3-6), but the nature of this essential function is not known. The CID is only found on RNA polymerase II (pol II), suggesting that it plays a unique role in mRNA biogenesis (7 Despite identification of interaction partners, the role of the CTD in transcription remains unclear. The CTD is not required for either basalV(14, 15) or activated (16, 17) transcription of some genes in vitro. Furthermore, inhibition of CTD kinase does not block in vitro transcription from the adenovirus major late promoter or from a GAL4 VP16-activated promoter (18,19). Thus, these results indicate that the CTD is not essential for specific initiation at some promoters.CTD function may be required for postinitiation steps in the biogenesis of mRNA. O'Brien et al. (20) have demonstrated that several genes contain paused pol IIA complexes that can reenter the elongation mode coincident with CTD phosphorylation. In yeast, CTD-truncated pol II synthesizes an excess of GAL4-induced promoter proximal transcripts (D. L. Bentley, personal communication). Thus, these results argue that the CTD plays an important role subsequent to initiation. While the CTD has previously been proposed to function in premRNA processing (refs. 7 and 21 and H. Rienhoff and J. Boeke, personal communication), no experimental data have yet supported these models.We used the yeast two-hybrid system (22) to identify proteins that interact with the CTD. This unbiased approach did not yield proteins that are expected to be involved in transcription initiation, like TBP or the SRBs, but rather a set of proteins similar to RNA processing factors. In this paper we report the identification and characte...

7 . Two forms of the largest subunit can be separated by SDS-polyacrylamide gel electrophoresis. The faster migrating form termed IIA contains little or no phosphate on the CTD, whereas the slower migrating II0 form is multiply phosphorylated. CTD kinases with different phosphoryl acceptor specificities are able to convert IIA to II0 in vitro, and different phosphoisomers have been identified in vivo. In this paper we report the binding specificities of a set of monoclonal antibodies that recognize different phosphoepitopes on the CTD. Monoclonal antibodies like H5 recognize phosphoserine in position 2, whereas monoclonal antibodies like H14 recognize phosphoserine in position 5. The relative abundance of these phosphoepitopes changes when growing yeast enter stationary phase or are heat-shocked. These results indicate that phosphorylation of different CTD phosphoacceptor sites are independently regulated in response to environmental signals.The largest subunit of RNA polymerase II (pol II) 1 contains a repetitive C-terminal domain (CTD) consisting of tandem repeats of the consensus sequence Tyr-Ser-Pro-Thr-Ser-ProSer (1, 2). The CTD plays an essential (3-6) but as yet poorly understood role in mRNA synthesis with evidence indicating potential roles in initiation or promoter clearance (7-9), elongation (10 -15), and pre-mRNA processing (16 -20).Phosphorylation of the CTD is a key feature of CTD function. SDS gel electrophoresis separates the largest subunit into two species as follows: IIA contains a hypophosphorylated CTD and pol II0 is hyperphosphorylated on the CTD (21). Serine is the predominant in vivo phosphoacceptor with minor amounts of phosphothreonine and phosphotyrosine detected (22, 23). Although in vivo phosphorylation sites have not been mapped, in vitro studies have identified serines in both positions 2 and 5 (22, 24, 25) and tyrosine in position 1 (23) as potential phosphoryl acceptors. Mutation of these sites to unphosphorylatable alanine or phenylalanine residues in each yeast CTD repeat is lethal, suggesting a requirement for CTD phosphorylation in vivo (26).The preferential inclusion of pol IIA into preinitiation complexes (27-30) together with the observation that elongating pol II is phosphorylated on the CTD (31) led to the hypothesis that the CTD is reversibly phosphorylated with each transcription cycle (8). The unphosphorylated CTD has been shown to contact basal transcription factors TATA binding protein (32), TFIIE, and TFIIF (33), and these contacts, together with as yet undefined interactions with SRBs (34 -37), suggest that the CTD acts as a structural framework for the preinitiation complex (38). The pol II preinitiation complex also contains several protein kinases that are capable of phosphorylating the CTD (39 -45) suggesting that one role of this complex is to effect the conversion of pol IIA to pol II0 thereby releasing pol II from the initiation complex. Finally, CTD phosphatase is required to dephosphorylate pol II0 thus completing the CTD phosphorylation cycle (46, 47).Se...

The P53 homolog p63 encodes multiple proteins with transactivating, apoptosis-inducing, and oncogenic activities. We showed that p63 is amplified and that DeltaNp63 isotypes are overexpressed in squamous cell carcinoma (SCC) and enhance oncogenic growth in vitro and in vivo. Moreover, p53 associated with DeltaNp63alpha and mediated its degradation. Here, we report that DeltaNp63 associates with the B56alpha regulatory subunit of protein phosphatase 2A (PP2A) and glycogen synthase kinase 3beta (GSK3beta), leading to a dramatic inhibition of PP2A-mediated GSK3beta reactivation. The inhibitory effect of DeltaNp63 on GSK3beta mediates a decrease in phosphorylation levels of beta-catenin, which induces intranuclear accumulation of beta-catenin and activates beta-catenin-dependent transcription. Our results suggest that DeltaNp63 isotypes act as positive regulators of the beta-catenin signaling pathway, providing a basis for their oncogenic properties.

A human p53 homologue, p63 (p40͞p51͞p73L͞CUSP) that maps to the chromosomal region 3q27-29 was found to produce a variety of transcripts that encode DNA-binding proteins with and without a trans-activation domain (TA-or ⌬N-, respectively). The p63 gene locus was found to be amplified in squamous cell carcinoma, and overexpression of ⌬Np63 (p40) led to increased growth of transformed cells in vitro and in vivo. Moreover, p63-null mice displayed abnormal epithelial development and germ-line human mutations were found to cause ectodermal dysplasia. We now demonstrate that certain p63 isotypes form complexes with p53. p53 mutations R175H or R248W abolish the association of p53 with p63, whereas V143A or R273H has no effect. Deletion studies suggest that the DNA-binding domains of both p53 and p63 mediate the association. Overexpression of wild type but not mutant (R175H) p53 results in the caspase-dependent degradation of certain ⌬Np63 proteins (p40 and ⌬Np63␣). The association between p53 and ⌬Np63 supports a previously unrecognized role for p53 in regulation of ⌬Np63 stability. The ability of p53 to mediate ⌬Np63 degradation may balance the capacity of ⌬Np63 to accelerate tumorigenesis or to induce epithelial proliferation.

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Yeast two-hybrid screening has led to the identification of a family of proteins that interact with the repetitive C-terminal repeat domain (CTD) of RNA polymerase II (A. Yuryev et al., Proc. Natl. Acad. Sci. USA 93: [6975][6976][6977][6978][6979][6980] 1996). In addition to serine/arginine-rich SR motifs, the SCAFs (SR-like CTD-associated factors) contain discrete CTD-interacting domains. In this paper, we show that the CTD-interacting domain of SCAF8 specifically binds CTD molecules phosphorylated on serines 2 and 5 of the consensus sequence Tyr 1 Ser 2 Pro 3 Thr 4 Ser 5 Pro 6 Ser 7 . In addition, we demonstrate that SCAF8 associates with hyperphosphorylated but not with hypophosphorylated RNA polymerase II in vitro and in vivo. This result suggests that SCAF8 is not present in preinitiation complexes but rather associates with elongating RNA polymerase II. Immunolocalization studies show that SCAF8 is present in granular nuclear foci which correspond to sites of active transcription. We also provide evidence that SCAF8 foci are associated with the nuclear matrix. A fraction of these sites overlap with a subset of larger nuclear speckles containing phosphorylated polymerase II. Taken together, our results indicate a possible role for SCAF8 in linking transcription and pre-mRNA processing.The carboxy-terminal domain (CTD) of the largest subunit of RNA polymerase II (pol II) consists of multiple repeats of the heptapeptide sequence YSPTSPS (2, 24). The CTD is found in pol II from a variety of species but is not present on pol I or pol III, indicating a role in mRNA biogenesis (22,23). Deletion studies have demonstrated that the CTD is essential for cell viability in yeast, Drosophila, and mammalian cells (3,4,55,75). While roles in transcription activation, enhancer function, and pre-mRNA processing have been proposed, the essential role of the CTD remains elusive.The CTD contains an uncommonly high density of potential phosphorylation sites (26,27). Both hyper-and hypophosphorylated pol II can be detected in vivo, but the distribution of phosphates among the more than 100 potential sites has not been determined. Substitution of alanine for serines in either position 2 or 5 of each repeat is lethal in yeast, suggesting that phosphorylation of both of these sites is essential (70).In vitro transcription experiments have led to the hypothesis that the CTD is reversibly phosphorylated with each transcription cycle (26,27). One role of phosphorylation may be to disrupt contacts between the unphosphorylated CTD and components of the holoenzyme. In addition, phosphorylation may prepare the CTD for a role subsequent to initiation. Indeed, transcription elongation is CTD dependent (1, 44) and is temporally linked to CTD phosphorylation (1,31,44,56,69).Yeast two-hybrid screens have led to the identification of a family of CTD-binding proteins containing serine/arginine-rich motifs (17,64,74). The SCAFs (SR-like CTD-associated factors) contain conserved N-terminal or C-terminal CTD-interacting domains (25). We have previo...