Smad transcription factors lie at the core of one of the most versatile cytokine signaling pathways in metazoan biology-the transforming growth factor-␤ (TGF␤) pathway. Recent progress has shed light into the processes of Smad activation and deactivation, nucleocytoplasmic dynamics, and assembly of transcriptional complexes. A rich repertoire of regulatory devices exerts control over each step of the Smad pathway. This knowledge is enabling work on more complex questions about the organization, integration, and modulation of Smad-dependent transcriptional programs. We are beginning to uncover self-enabled gene response cascades, graded Smad response mechanisms, and Smad-dependent synexpression groups. Our growing understanding of TGF␤ signaling through the Smad pathway provides general principles for how animal cells translate complex inputs into concrete behavior.As evolution unfolded and multicellular life forms emerged, so did the need for tight control over the ability of individual cells to move, divide, differentiate, and organize. Intricate intercellular communication systems evolved to ensure the proper behavior of individual cells in the context of the whole organism. Among these forms of communication, one of the most prevalent involves secretory polypeptides that are recognized by membrane receptors coupled to transcriptional regulatory factors. With its 42 members in the human genome, seven in Drosophila, and four in Caenorhabditis elegans, the transforming growth factor-␤ (TGF␤) family is one of the most prominent representatives of this class of molecules. TGF␤ and its family members-the nodals, activins, bone morphogenetic proteins (BMPs), myostatins, anti-Muellerian hormone (AMH), and others-exert profound effects on cell division, differentiation, migration, adhesion, organization, and death.

Following TGFbeta receptor-mediated phosphorylation and association with Smad4, Smad2 moves into the nucleus, binds to target promoters in association with DNA-binding cofactors, and recruits coactivators such as p300/CBP to activate transcription. We identified the homeodomain protein TGIF as a Smad2-binding protein and a repressor of transcription. A TGFbeta-activated Smad complex can recruit TGIF and histone deacetylases (HDACs) to a Smad target promoter, repressing transcription. Thus, upon entering the nucleus, a Smad2-Smad4 complex may interact with coactivators, forming a transcriptional activation complex, or with TGIF and HDACs, forming a transcriptional repressor complex. Formation of one of these two mutually exclusive complexes is determined by the relative levels of Smad corepressors and coactivators within the cell.

Polycomb group (PcG) proteins form large multimeric complexes (PcG bodies) which are involved in the stable repression of gene expression. The human PcG protein, Pc2, has been shown to recruit the transcriptional corepressor, CtBP, to PcG bodies. We show that CtBP is sumoylated at a single lysine. In vitro, CtBP sumoylation minimally requires the SUMO E1 and E2 (Ubc9) and SUMO-1. However, Pc2 dramatically enhances CtBP sumoylation. In vivo, this is likely due to the ability of Pc2 to recruit both CtBP and Ubc9 to PcG bodies, thereby bringing together substrate and E2, and stimulating the transfer of SUMO to CtBP. These results demonstrate that Pc2 is a SUMO E3, and suggest that PcG bodies may be sumoylation centers.

The Saccharomyces cerevisiae Rapl protein binds with high affinity to sites within the poly(Ci_3A) tracts at telomeres, where it plays a role in both telomere length regulation and the initiation of telomeric silencing. Raplp initiates silencing at telomeres by interacting through its carboxy-terminal domain with Sir3p and Sir4p, both of which are required for repression. This same domain of Raplp also negatively regulates telomere elongation, through an unknown mechanism. We have identified a new Rapl-interacting factor (Rif2p) that plays a role in telomere length regulation. RifZp has considerable functional similarities with a Raplp-interacting factor (Riflp) identified previously. Mutations in RIFl or RJF2 (unlike mutations in the silencing genes SIRS and SIR4) result in moderate telomere elongation and improved telomeric silencing. However, deletion of both RIFl and RIF2 in the same cell results in a dramatic increase in telomere length, similar to that seen with a carboxy-terminal truncation of Raplp. In addition, overexpression of either jRIFl or RIF2 decreases telomere length, and co-overexpression of these proteins can reverse the telomere elongation effect of overexpression of the Raplp carboxyl terminus. Finally, we show that Riflp and Rif2p can interact with each other in vivo. These results suggest that telomere length regulation is mediated by a protein complex consisting of Riflp and Rif2p, each of which has distinct regulatory functions. One role of Raplp in telomere length regulation is to recruit these proteins to the telomeres.