Bonus, a Drosophila homolog of TIF1 proteins, interacts with nuclear receptors and can inhibit betaFTZ-F1-dependent transcription

Abstract

The Drosophila bonus (bon) gene encodes a homolog of the vertebrate TIF1 transcriptional cofactors. bon is required for male viability, molting, and numerous events in metamorphosis including leg elongation, bristle development, and pigmentation. Most of these processes are associated with genes that have been implicated in the ecdysone pathway, a nuclear hormone receptor pathway required throughout Drosophila development. Bon is associated with sites on the polytene chromosomes and can interact with numerous Drosophila nuclear receptor proteins. Bon binds via an LxxLL motif to the AF-2 activation domain present in the ligand binding domain of betaFTZ-F1 and behaves as a transcriptional inhibitor in vivo.

Figure 1. bon Plays a Role in Several Developmental Events and Is Required for Organismal Viability

(A) Transcriptional responses to the larval and the prepupalecdysone pulses. Time points are shown hours prior to and after pupariation.(B) Animals with the listed genotypes were analyzed for their phase of lethality. The percentage of animals that died at each developmental stage is shown graphically (n = 200).(C and D) Animals dissected from the pupal case at ~4 days post-pupariation (pharate adult). (C) y w pharate adult compared to a similarly aged (D) y w; bon⁴⁸⁷/bon^21B pupa initiate but fail to complete development of legs (outlined), wings, head, eyes, and cuticle.(E and F) Salivary glands of y w third instar larva and y w; bon⁴⁸⁷/bon^21B pupa ( days post pupariation).(G) y w; bon²⁴¹/bon⁴⁸⁷ pharate adult dissected from its pupal cases 4 days post-pupariation. Defects are seen in cuticle morphology, pigmentation of the wing (blue asterisk), and loss of tergite (yellow arrow) and sternal (white arrow) bristles (compare with Figure 1C).(H and I) y w and y w; bon²⁴¹/bon⁴⁸⁷ wings. (I) y w; bon²⁴¹/bon⁴⁸⁷ wings display a loss of bristles along the anterior wing margin (black arrow).(J and K) Adult heads of y w; FRT82B w⁺ clR3 (J) and y w eyeless-FLP glass-lacZ; FRT82B w⁺ clR3/FRT82B bon^21B (K) flies. Note the severe loss of photoreceptors and cuticle.

Figure 2. bon Encodes the Drosophila Homolog of Vertebrate TIF1

(A) Approximately 20 kb of genomic DNA covering the bon locus (E = EcoRI restriction sites) is shown graphically. bon²⁴¹ and bon²⁴⁹ P-elements map to the 5′ end of exon 1 and bon⁴⁸⁷ P-element maps to the middle of intron 1. bon^21B removes all but 12 bp of exon 1 and the first 324 bp of intron 1.(B) Domain structures and homology between Bon and mouse TIF1α. Schematics of both proteins show conserved domains.(C) Rescue of bon mutations. A bon cDNA under the control of a Hsp83 ubiquitously expressing promoter rescues the bon²⁴¹/bon⁴⁸⁷ phenotypes and lethality as observed in the increased percentage of both mutant pupae and adults (n = 400).

Figure 3. Bon Is a Nuclear Protein that Is Expressed throughout Development

(A) Developmental Northern analysis of bon expression hours after egg lay.(B) Developmental Northern and Western analysis of bon mRNA and protein hours before and after pupariation. rp49 serves as a loading control.(C and D) Stage 17 embryos stained with anti-Bon antibody (green) and the neuronal specific MAB 22C10 (red). White arrows point to ectopic neurons in the dorsal and ventral clusters.(E) Western analysis of bon^21B/+ (control) and bon^21B/bon⁴⁸⁷ 4 hr prepupae.(F–K) Immunohistochemistry using anti-Bon antibody (red) and DAPI (blue). Bon is found in fat body (F), wing disc (G), an eye/antennal disc (H), leg disc (I), and salivary gland (J) from a prepupa. Bon localization on polytene chromosome 2R (K) of a late third instar larva salivary gland.

Figure 4. Ecdysone-Induced Genes Are Misregulated in bon

Total RNA was isolated from staged y w control and bon²⁴¹/bon²⁴¹ mutant animals and analyzed by Northern. Numbers on the top indicate hours pre and post pupariation at 25°C. In the bon²⁴¹/bon²⁴¹ background bon transcripts are severely reduced, although some transcripts of higher molecular weight, due to the P-element insertion, are detected. In comparison to y w, DHR3 transcripts are upregulated in the bon²⁴¹/bon²⁴¹ animals. βFTZ-F1, EcRA, EcRB, E74A, E74B, and BR-C transcript levels are reduced in the bon²⁴¹/bon²⁴¹ as compared to y w control background. Levels of rp49 transcripts serve as a control.

Figure 5. Bon Interacts with Several Nuclear Receptors and Selectively Interacts with the AF-2 AD Core Motif of βFTZ-F1 through a Conserved LxxLL Motif

(A) Bon interacts with βFTZ-F1 (amino acids 315–802) in yeast. Plasmids expressing the unfused ERα DBD or DBD-Bon were introduced into PL3 together with the unfused VP16 AAD or AAD–βFTZ-F1 (315–802). Transformants were grown in liquid medium containing uracil. Extracts were prepared and assayed for OMP decase activity (nmol substrate/min/mg protein).(B) Bon interacts directly with full-length βFTZ-F1 and other Drosophila nuclear receptors in vitro. His-tagged Bon was incubated with control GST (lane 2) or GST fusions containing the indicated nuclear receptors (lanes 3–5) bound to glutathione-S-Sepharose beads. Bound Bon was detected by Western. Lane 1 shows one-fifth of the amount of input His-Bon.(C) Schematic representation of the structure of βFTZ-F1 with conserved regions of the receptor denoted A–E. Amino acid boundaries (numbers) were defined by sequence alignment with other nuclear receptors (Wurtz et al., 1996). A comparative alignment of the FTZ-F1 AF-2 AD core motif with other AF-2 AD core sequences is shown with the highly conserved pairs of hydrophobic residues boxed and charged residues shaded.(D) Bon interacts with the LBD of βFTZ-F1 in an AF-2-dependent manner. The indicated regions of βFTZ-F1 were fused to the VP16 AAD (hatched box) and assayed for activation with DBD-Bon in PL3. OMP decase activities are expressed as in Figure 5A. The values (±20%) are the average of at least three independent transformants. Expression of fusion proteins was confirmed by Western blotting using the antibody 2GV4 against VP16 (data not shown).(E) Characterization of the AF-2 activation domain of βFTZ-F1. S2 cells were transfected with a reporter construct containing a GAL4 binding site (17M-ERE-tk-CAT) and pCMV-β-gal, with or without vectors containing unfused GAL4 (control) or GAL4 fused to βFTZ-F1 (555–802) wild-type (WT) or to βFTZ-F1 (555–802)Δ791–797 lacking the AF-2 AD core motif (ΔAF-2AD core). CAT activities are expressed relative to that of the unfused GAL4 vector (taken as 100%). Values (±10%) represent the averages of three independent transfections after normalization to β-gal activities.(F) The LxxLL motif of Bon is both necessary and sufficient for interaction with the LBD/AF-2 of βFTZ-F1. The indicated mutants of Bon were fused to the ERαDBD and assayed for interaction with the “unfused” AAD or AAD fusions containing the entire E region of βFTZ-F1 (amino acids 555–802; AAD–βFTZ-F1 [E] WT) or a mutated E derivative in which the core of the AF-2 AD was deleted (Δ791 to 797; AAD–βFTZ-F1 [E] ΔAF-2AD core). OMPdecase activities are given in nmol substrate/min/mg protein. The values (±20%) are the average of at least three independent transformants. Expression of all the DBD and AAD fusion proteins indicated was confirmed by Western blotting using the antibodies F3 against the F region of ERα and 2GV4 against VP16, respectively (data not shown).

Figure 6. Loss of Bon Suppresses Phenotypes Associated with βFTZ-F1

(A) Df(3L)Cat^DH104/+ (Df/+) control animal prior to eclosion.(B–D) Df(3L)Cat^DH104/βFTZ-F1^ex17 mutant animals four days after pupariation: (B) Pharate adult stage with malformed legs.(C) Pupa that has completed head eversion, but arrested in development.(D) Cryptocephalic pharate adult with malformed legs.(E) Shows the percentage of animals of listed genotypes that are adults (red), pharate adults (yellow), arrested pupae (green), or cryptocephalic adults (blue) (n = 400). ex17 = βFTZ-F1^ex17, Df = Df(3L)Cat^DH104, 487 = bon⁴⁸⁷, 241 = bon²⁴¹.(F) Northern blot analysis of βFTZ-F1^ex17/+, βFTZ-F1^ex17/βFTZ-F1^ex17, and βFTZ-F1^ex17, bon⁴⁸⁷/βFTZ-F1^ex17 animals at 6, 8, and 10 hr after pupariation.

Figure 7. Bon Has an Autonomous Repression Function and Negatively Regulates Activation by βFTZ-F1 AF-2

(A) Bon represses activated transcription in a dose-dependent manner when targeted to the promoter. The 17M-ERE-tk-CAT (2 μg) reporter and 2 μg of pCMV–β–gal plasmid (as internal control) were cotransfected into S2 cells with the indicated pPAC5C-based vectors expressing the activator ER(C)-VP16 (2 μg) and the unfused GAL4 DBD (GAL4: 2 μg) or Bon fused to the GAL4 DBD (GAL4-Bon: 0.02, 0.2, or 2 μg) or Bon tagged with the FLAG epitope (Bon: 2 μg). CAT activities are expressed relative to controls of ER(C)-VP16 and the unfused GAL4 (taken as 100%). Values (±10%) represent the averages of three independent duplicated transfections after normalization to β-galactosidase activities.(B) The RBCC motif of Bon is required for repression. Cotransfection assays were conducted as described in (A) with GAL4-Bon expression vector. Expression of the fusion proteins was confirmed by Western using the antibody 2GV3 against the GAL4 DBD (data not shown).(C) Effect of Bon overexpression on transactivation by βFTZ-F1 AF-2. S2 cells were transfected with the 17M-ERE-tk-CAT reporter plasmid (3 μg), 2 μg of pCMV–β–gal plasmid, and 6 μg of GAL4 expression vectors encoding GAL4 or GAL4–βFTZ-F1 (E), together with 1, 3, and 6 μg of either FLAG-tagged Bon wild-type or FLAG-tagged Bon (471–890). CAT activities are expressed relative to those of GAL4–βFTZ-F1 (E) (taken as 100%). Values (±0%) represent the averages of three independent duplicated transfections after normalization to β–gal activities.