The Pipsqueak Protein of Drosophila melanogasterBinds to GAGA Sequences through a Novel DNA-binding Domain

Abstract

Pipsqueak (Psq) belongs to a family of proteins defined by a phylogenetically old protein-protein interaction motif. Like the GAGA factor and other members of this family, Psq is an important developmental regulator in Drosophila, having pleiotropic functions during oogenesis, embryonic pattern formation, and adult development. The GAGA factor controls the transcriptional activation of homeotic genes and other genes by binding to control elements containing the GAGAG consensus motif. Binding is associated with formation of an open chromatin structure that makes the control regions accessible to transcriptional activators. We show here that Psq contains a novel DNA-binding domain, which binds, like the GAGA factor zinc finger DNA-binding domain, to target sites containing the GAGAG consensus motif. Binding is suppressed, as in the GAGA factor and other proteins of the family, by the associated protein-protein interaction motif. The DNA-binding domain, which we call the Psq domain, is identical with a previously identified region consisting of four tandem repeats of a conserved 50-amino acid sequence, the Psq motif. The Psq domain seems to be structurally related to known DNA-binding domains, both in its repetitive character and in the putative three-␣-helix structure of the Psq motif, but it lacks the conserved sequence signatures of the classical eukaryotic DNA-binding motifs. Psq may thus represent the prototype of a new family of DNA-binding proteins. Members of the BTB1 /POZ protein family play important roles in development and reproduction of Drosophila melanogaster. These proteins contain a protein-protein interaction motif that was first identified in zinc finger proteins encoded by the Drosophila Broad-Complex and tramtrack genes (1, 2), and later also in the bric à brac gene product (3). The domain, which was thereupon designated as BTB (Broad-Complex, Tramtrack, Bric à brac) domain (3), has since been found in proteins of a variety of species, as diverse as slime molds (4) and humans (for a review, see Ref. 5). Many of these proteins are DNA-binding C 2 H 2 zinc finger proteins, but the presence of the domain in a family of pox virus proteins (6) soon indicated that coupling to a DNA-binding domain is not mandatory. The domain is therefore also referred to as the POZ (pox virus, zinc finger) domain (7). In BTB/POZ proteins that contain a zinc finger DNA-binding motif, DNA binding is strongly inhibited by the BTB/POZ domain. This inhibitory effect on DNA binding is also observed in chimeric proteins in which the BTB/POZ domain is associated with a heterologous DNA-binding domain, for instance a POU domain (7). Inhibition of DNA binding appears to be the result of oligomerization through proteinprotein interactions mediated by the BTB/POZ domain.The tendency of BTB/POZ proteins to oligomerize in solution and their localization in distinct nuclear substructures (7-10) suggests that they might act by modifying chromatin structure (5). In fact, such a mode of action is supported by different lines of eviden...

Ligand-dependent Corepressor LCoR Is an Attenuator of Progesterone-regulated Gene Expression

“…The LCoR HTH domain also bears 35% homology to pipsqueak motifs (PSQ). PSQ motifs are repeated four times in the DNA binding region of the Drosophila pipsqueak transcription factor, which plays a role in gene silencing (31). Multiple repeats of the domain are required for PSQ DNA binding (31), and mutation of one of the two HTH motifs in the MBLK-1 gene strongly reduced sitespecific DNA binding (30).…”

“…PSQ motifs are repeated four times in the DNA binding region of the Drosophila pipsqueak transcription factor, which plays a role in gene silencing (31). Multiple repeats of the domain are required for PSQ DNA binding (31), and mutation of one of the two HTH motifs in the MBLK-1 gene strongly reduced sitespecific DNA binding (30). The PSQ domain is homologous to unique motifs found in a number of prokaryotic and eukaryotic proteins that interact with DNA, such as recombinases (31,32), raising the possibility that LCoR itself may interact with DNA.…”

“…Multiple repeats of the domain are required for PSQ DNA binding (31), and mutation of one of the two HTH motifs in the MBLK-1 gene strongly reduced sitespecific DNA binding (30). The PSQ domain is homologous to unique motifs found in a number of prokaryotic and eukaryotic proteins that interact with DNA, such as recombinases (31,32), raising the possibility that LCoR itself may interact with DNA. Other studies have shown that HTH domains can function in protein-protein interactions, where the HTH motif, combined with other domains, can induce formation of multisubunit complexes (33).…”

Expression pattern of Gal4 enhancer trap insertions into the bric à brac locus generated by P element replacement

“…bab is a large locus covering 140 kb and is composed of two genes, bab1 and bab2, that are similar in sequence and structure and likely arose by a chromosomal duplication (Couderc et al, 2002). Both genes encode proteins with a BTB domain and a pipsqueak motif (Couderc et al, 2002), domains found in a number of transcriptional regulators (Collins, 2001;Lehmann, 1998). Both Bab1 and Bab2 are localized to the nucleus (Couderc et al, 2002;Godt and Laski, 1995;Godt et al, 1993) and interact with BIP2/ TAF II 155, a component of TFIID (Pointud et al, 2001).…”

Expressing UAS‐bab1 and UAS‐bab2: A comparative study of gain‐of‐function effects and the potential to rescue the bric à brac mutant phenotype

“…It contains two structurally related genes, bric à brac1 (bab1) and bric à brac2 (bab2) (Couderc et al, 2002) that encode nuclear proteins sharing two strongly conserved domains, an N-terminal BTB domain and a Cterminal Psq domain (Godt et al, 1993;Zollman et al, 1994;Couderc et al, 2002). Both domains have been found in a number of transcriptional regulators (Collins et al;Lehmann et al;1998). The Bab1 and Bab2 proteins interact with BIP2/TAFII155/TAF3, a component of TFIID (Pointud et al, 2001), suggesting a function in transcriptional regulation by directly interacting with the basal transcriptional machinery.…”