(Gro) and C-terminal binding protein (CtBP), function as part of complexes containing enzymes that influence transcription by covalently modifying histones and influencing nucleosome packing and the binding of chromatin-associated proteins

INTRODUCTION Transcriptional repressors commonly recruit accessory proteins known as co-repressors (CoRs) that provide them with repressive activity by modifying chromatin structure. CoRs, such as Groucho (Gro) and C-terminal binding protein (CtBP), function as part of complexes containing enzymes that influence transcription by covalently modifying histones and influencing nucleosome packing and the binding of chromatin-associated proteins (Chen et al., 1999; Gromöller and Lehming, 2000; Zhang and Emmons, 2002; Shi et al., 2003; Subramanian and Chinnadurai, 2003; Kim et al., 2005; Winkler et al., 2010). Theoretically, the recruitment of a single CoR could be sufficient for a repressor to silence all of its target genes. However, many repressors can recruit more than one CoR; for example, the Drosophila repressors Hairy, Hairless, Knirps and Brinker (Brk) can each recruit CtBP and Gro via conserved 4-10 amino acid CtBPand Gro-interaction motifs (CiMs and GiMs) (Paroush et al., 1994; Nibu et al., 1998a; Poortinga et al., 1998; Hasson et al., 2001; Zhang et al., 2001; Barolo et al., 2002; Payankaulam and Arnosti, 2009). This ability to recruit both CoRs is somewhat perplexing given that they appear to possess different properties, in particular in respect to the distance over which they can function, with CtBP activity being limited to short distances of ~150 bp from a transcription factor (TF) binding site (Nibu et al., 1998a), whereas Gro can function over a much longer range (Barolo and Levine, 1997; Martinez and Arnosti, 2008); although when recruited by Knirps, Gro has similar short-range properties to CtBP (Payankaulam and Arnosti, 2009). Consequently, it is unclear what CtBP can do that Gro cannot, raising the question of why Gro alone is not sufficient? Possible reasons are as follows. (1) Quantitative: two CoRs may additively provide more repressive activity than can be provided by one alone. (2) Qualitative: one CoR may provide a unique activity that is not provided by the other and which is essential for repression of one or more target genes. Alternatively, a TF may be unable to recruit one CoR at some targets where the other would be required. (3) To minimize noise: a second CoR may serve as a backup to ensure that the TF works efficiently all the time. (4) Availability: each CoR may not be expressed or active in all cells in which the TF functions. Both CtBP and Gro appear to be expressed ubiquitously (Nibu et al., 1998b; Poortinga et al., 1998; Jennings and IshHorowicz, 2008) but Gro activity can be downregulated by phosphorylation downstream of receptor tyrosine kinase (RTK) signaling cascades (Hasson et al., 2005; Cinnamon et al., 2008). Previous studies on the TFs Hairy, Hairless, Knirps and Brk have not been conclusive in uncovering why they possess recruitment motifs for both Gro and CtBP. Most studies reveal that Gro is essential for the repression of at least some targets; for example, reducing Gro levels results in derepression of the Hairless, Hairy, Knirps and Brk targets vgQE, fushi tarazu, even skipped and spalt [sal; spalt major (salm) – FlyBase], respectively (Paroush et al., 1994; Winter and Campbell, 2004; Nagel et al., 2005; Jennings et al., 2008; Payankaulam and Arnosti, 2009). Reducing CtBP levels often has no effect, for example on Brk or Hairless targets in the wing disc (Winter and Campbell, 2004; Nagel et al., 2005), although some Knirps targets and to a lesser extent Hairy targets may show derepression during embryogenesis (Keller et al., 2000; Bianchi-Frias et al., 2004; Struffi and Arnosti, 2005). In some cases, a TF can repress some of its targets even in the absence of both Gro and CtBP, as is the case for the Brk target optomotor-blind (omb; bifid – FlyBase) in the wing disc, indicating that they can use additional mechanisms to repress; Brk has a third repression domain, 3R (Winter and Campbell, 2004). Brk may also recruit a third CoR, Nab, although this does not appear to be required for growth and patterning, but only for Brk-dependent apoptosis induced by reduced Dpp signaling (Ziv et al., 2009). Department of Biological Sciences, University of Pittsburgh, Pittsburgh, PA 15260, USA.