Cullin3 is a KLHL10-interacting protein preferentially expressed during late spermiogenesis

Kelch-like 10 (KLHL10) is a member of the BTB (Bric-a-brac, Tramtrack, and Broad-Complex)-kelch protein superfamily essential for spermiogenesis and male fertility. In a search for KLHL10-interacting proteins using a yeast two-hybrid assay, we identified Cullin3 (CUL3) as one of multiple KLHL10-interacting partners. Yeast cotransformation assays revealed that CUL3 bound the BTB/POZ domain of KLHL10. Northern blot and quantitative RT-PCR analyses demonstrated that Cul3 mRNA was preferentially expressed in the testis. In situ hybridization analysis localized Cul3 mRNA to spermatids in the adult testis. CUL3 protein was detected in elongating and elongated spermatids (steps 10-16) by immunofluorescent microscopy. The expression pattern of CUL3 resembles KLHL10. CUL3 was coimmunoprecipated with KLHL10, and KLHL10 was also detected in the CUL3 immunoprecipitants using testis lysates. These findings suggest that KLHL10, like other BTB/kelch proteins, interacts with CUL3 to form a CUL3-based ubiquitin E3 ligase that functions specifically in the testis to mediate protein ubiquitination during spermiogenesis.     

In vivo function of a novel Siah protein in Drosophila

The Siah proteins, mammalian homologues of the Drosophila Sina protein, function as E3 ubiquitin ligase enzymes and target a wide range of cellular proteins for degradation. Here, I investigate the in vivo function of the fly protein, Sina-Homologue (SinaH), which is highly similar to Sina. Flies that completely lack SinaH are viable and in combination with a mutation in the gene, Ebi, show an extra dorsal central bristle phenotype. I also show that SinaH and Ebi can interact with each other both in vivo and in vitro suggesting that they act in the same physical complex. Flies that lack both Sina and Sina-Homologue were also created and show visible eye and bristle phenotypes, which can be explained by an inability to degrade the neuronal repressor, Tramtrack. I find no evidence for redundancy in the function of Sina and SinaH.     

Mechanism of cullin3 e3 ubiquitin ligase dimerization

Cullin E3 ligases are the largest family of ubiquitin ligases with diverse cellular functions. One of seven cullin proteins serves as a scaffold protein for the assembly of the multisubunit ubiquitin ligase complex. Cullin binds the RING domain protein Rbx1/Rbx2 via its C-terminus and a cullin-specific substrate adaptor protein via its N-terminus. In the Cul3 ubiquitin ligase complex, Cul3 substrate receptors contain a BTB/POZ domain. Several studies have established that Cul3-based E3 ubiquitin ligases exist in a dimeric state which is required for binding of a number of substrates and has been suggested to promote ubiquitin transfer. In two different models, Cul3 has been proposed to dimerize either via BTB/POZ domain dependent substrate receptor homodimerization or via direct interaction between two Cul3 proteins that is mediated by Nedd8 modification of one of the dimerization partners. In this study, we show that the majority of the Cul3 proteins in cells exist as dimers or multimers and that Cul3 self-association is mediated via the Cul3 N-terminus while the Cul3 C-terminus is not required. Furthermore, we show that Cul3 self-association is independent of its modification with Nedd8. Our results provide evidence for BTB substrate receptor dependent Cul3 dimerization which is likely to play an important role in promoting substrate ubiquitination.     

pipsqueak encodes a novel nuclear protein required downstream of seven-up for the development of photoreceptors R3 and R4.

Photoreceptor induction in the Drosophila eye is mediated by activation of the Ras signal transduction cascade. Although this process is well understood, little is known about how the diversity of photoreceptor subtypes is generated. The pipsqueak (psq) gene is expressed at high levels in the R3/R4 precursors during eye development and this expression depends on seven-up (svp) gene function. Moreover, strong psq alleles are dominant suppressors of a svp-induced cone cell transformation phenotype. Although the gene was previously identified and described as a member of the maternal posterior group of genes, the strong semilethal alleles isolated here demonstrate a specific requirement for psq function downstream of svp for the development of photoreceptors R3/R4. The gene has three independent 5' ends and codes for several nuclear protein isoforms, some containing the POZ domain which has been implicated in protein-protein interactions. Interestingly, all viable alleles with a maternal posterior group phenotype cluster around one specific 5' exon, while all semilethal alleles have lesions which map to a different alternative 5' exon.     

A ubiquitin-protein ligase specific for type III protein substrates

A previously studied species of ubiquitin-protein ligase contains specific sites for the binding of basic (Type I) and bulky hydrophobic (Type II) NH2-terminal amino acid residues of protein substrates. We now describe another enzyme that ligates ubiquitin specifically to proteins that have NH2-terminal residues other than the above two categories (Type III substrates). The new species of ligase, that we call E3 beta, is separable from the formerly described ligase (termed E3 alpha) by affinity chromatography on protein substrate columns. E3 beta was partially purified from extracts of rabbit reticulocytes and was shown to be required for the breakdown of Type III proteins. Apart from its different substrate specificity, it resembles E3 alpha in some physical properties, in a requirement for ubiquitin carrier protein (E2) for conjugate formation, and in its action to ligate multiple ubiquitin units to the substrate protein. The denatured derivative of bovine pancreatic ribonuclease is a specific substrate for E3 alpha, while that of ribonuclease S-protein is a good substrate for E3 beta. Since S-protein is formed by the removal from ribonuclease of NH2-terminal S-peptide, it is suggested that E3 beta interacts with an NH2-terminal determinant exposed in ribonuclease S-protein.     

Molecular organization of the cullin E3 ligase adaptor KCTD11

The family of human proteins containing a potassium channel tetramerization domain (KCTD) includes 21 members whose function is largely unknown. Recent reports have however suggested that these proteins are implicated in very important biological processes. KCTD11/REN, the best-characterized member of the family to date, plays a crucial role in the ubiquitination of HDAC1 by acting, in complex with Cullin3, as an E3 ubiquitin ligase. By combining bioinformatics and mutagenesis analyses, here we show that the protein is expressed in two alternative variants: a short previously characterized form (sKCTD11) composed by 232 amino acids and a longer variant (lKCTD11) which contains an N-terminal extension of 39 residues. Interestingly, we demonstrate that lKCTD11 starts with a non-canonical AUU codon. Although both sKCTD11 and lKCTD11 bear a POZ/BTB domain in their N-terminal region, this domain is complete only in the long form. Indeed, sKCTD11 presents an incomplete POZ/BTB domain. Nonetheless, sKCTD11 is still able to bind Cul3, although to much lesser extent than lKCTD11, and to perform its biological activity. The heterologous expression of sKCTD11 and lKCTD11 and their individual domains in Escherichia coli yielded soluble products as fusion proteins only for the longer form. In contrast to the closely related KCTD5 which is pentameric, the characterization of both lKCTD11 and its POZ/BTB domain by gel filtration and light scattering indicates that the protein likely forms stable tetramers. In line with this result, experiments conducted in cells show that the active protein is not monomeric. Based on these findings, homology-based models were built for lKCTD11 BTB and for its complex with Cul3. These analyses indicate that a stable lKCTD11 BTB-Cul3 three-dimensional model with a 4:4 stoichiometry can be generated. Moreover, these models provide insights into the determinants of the tetramer stability and into the regions involved in lKCTD11-Cul3 recognition.     

c-MIR,a human E3 ubiquitin ligase,is a functional homolog of herpesvirus proteins MIR1 and MIR2 and has similar activity

Kaposi's sarcoma associated-herpes virus encodes two proteins, MIR (modulator of immune recognition) 1 and 2, which are involved in the evasion of host immunity. MIR1 and 2 have been shown to function as an E3 ubiquitin ligase for immune recognition-related molecules (e.g. major histocompatibility complex class I, B7-2, and ICAM-1) through the BKS (bovine herpesvirus 4, Kaposi's sarcoma associated-herpes virus, and Swinepox virus) subclass of plant homeodomain (PHD) domain, termed the BKS-PHD domain. Here we show that the human genome also encodes a novel BKS-PHD domain-containing protein that functions as an E3 ubiquitin ligase and whose putative substrate is the B7-2 co-stimulatory molecule. This novel E3 ubiquitin ligase was designated as c-MIR (cellular MIR) based on its functional and structural similarity to MIR1 and 2. Forced expression of c-MIR induced specific down-regulation of B7-2 surface expression through ubiquitination, rapid endocytosis, and lysosomal degradation of the target molecule. This specific targeting was dependent upon the binding of c-MIR to B7-2. Replacing the BKS-PHD domain of MIR1 with the corresponding domain of c-MIR did not alter MIR1 function. The discovery of c-MIR, a novel E3 ubiquitin ligase, highlights the possibility that viral immune regulatory proteins originated in the host genome and presents unique functions of BKS-PHD domain-containing proteins in mammals.     

BTB/POZ domain proteins are putative substrate adaptors for cullin 3 ubiquitin ligases

Cullins (CULs) are subunits of a prominent class of RING ubiquitin ligases. Whereas the subunits and substrates of CUL1-associated SCF complexes and CUL2 ubiquitin ligases are well established, they are largely unknown for other cullin family members. We show here that S. pombe CUL3 (Pcu3p) forms a complex with the RING protein Pip1p and all three BTB/POZ domain proteins encoded in the fission yeast genome. The integrity of the BTB/POZ domain, which shows similarity to the cullin binding proteins SKP1 and elongin C, is required for this interaction. Whereas Btb1p and Btb2p are stable proteins, Btb3p is ubiquitylated and degraded in a Pcu3p-dependent manner. Btb3p degradation requires its binding to a conserved N-terminal region of Pcu3p that precisely maps to the equivalent SKP1/F box adaptor binding domain of CUL1. We propose that the BTB/POZ domain defines a recognition motif for the assembly of substrate-specific RING/cullin 3/BTB ubiquitin ligase complexes.     

The KLHL12-Cullin-3 ubiquitin ligase negatively regulates the Wnt-beta-catenin pathway by targeting Dishevelled for degradation

Dishevelled is a conserved protein that interprets signals received by Frizzled receptors. Using a tandem-affinity purification strategy and mass spectrometry we have identified proteins associated with Dishevelled, including a Cullin-3 ubiquitin ligase complex containing the Broad Complex, Tramtrack and Bric à Brac (BTB) protein Kelch-like 12 (KLHL12). This E3 ubiquitin ligase complex is recruited to Dishevelled in a Wnt-dependent manner that promotes its poly-ubiquitination and degradation. Functional analyses demonstrate that regulation of Dishevelled by this ubiquitin ligase antagonizes the Wnt-beta-catenin pathway in cultured cells, as well as in Xenopus and zebrafish embryos. Considered with evidence that the distinct Cullin-1 based SCF(beta-TrCP)complex regulates beta-catenin stability, our data on the stability of Dishevelled demonstrates that two distinct ubiquitin ligase complexes regulate the Wnt-beta-catenin pathway.     

Guanine-nucleotide binding activity, interaction with GTPase-activating protein and solution conformation of the human c-Ha-Ras protein catalytic domain are retained upon deletion of C-terminal 18 amino acid residues

A trucated human c-Ha-Ras protein that lacks the C-terminal 18 amino acid residues and the truncated Ras protein with the amino acid substitution Gly Val in position 12 were prepared by anE. coli overexpression system. The truncated Ras protein showed the same guanine-nucleotide binding activity and GTPase activity as those of the full-length Ras protein. Further, the same extent of GTPase activity enhancement due to GTPase-activating protein was observed for the truncated and full-length Ras proteins. In fact, two-dimensional proton NMR analyses indicated that the tertiary structure of the truncated Ras protein (GDP-bound or GMPPNP-bound) was nearly the same as that of the corresponding catalytic domain of the full-length Ras protein. Moreover, a conformational change around the effector region upon GDP GMPPNP exchange occurred in the same manner for both proteins. These observations indicate that the C-terminal flanking region (18 amino acid residues) of the Ras protein does not appreciably interact with the catalytic domain. Therefore, the truncated Ras protein is suitable for studying the molecular mechanism involved in the GTPase activity and the interaction with the GTPase-activating protein. On the other hand, an active form of the truncated Ras protein, unlike that of the full-length Ras protein, did not induce neurite outgrowth of rat pheochromocytoma PC12 cells. Thus, membrane anchoring of the Ras protein through its C-terminal four residues is not required for the interaction of Ras and GAP, but may be essential for the following binding of the Ras-GAP complex with the putative downstream target.     

Characterization of the Cullin7 E3 ubiquitin ligase--heterodimerization of cullin substrate receptors as a novel mechanism to regulate cullin E3 ligase activity

Cul1 and Cul7 are cullin E3 ubiquitin ligase scaffold proteins. Cul1 is known to form a complex with the RING domain protein Rbx1 and one of approximately 70 different F-box proteins. F-box proteins function as substrate receptor subunits and recruit numerous substrates for poly-ubiquitination. Similarly to Cul1, Cul7 interacts with Rbx1, however, only one F-box protein, Fbxw8, has been shown to bind to Cul7. To date only few Cul7 E3 ubiquitin ligase substrates, including cyclin D1, IRS-1 and GRASP65, have been reported, and using Fbxw8 affinity purification, we were unable to identify additional substrate proteins. Here we provide evidence for a model in which Cul7-Rbx1 can promote the ubiquitination of Cul1 substrates by forming high order complexes with Cul1-Rbx1. Binding of Cul1-Rbx1 to Cul7-Rbx1 is mediated via heterodimerization of Fbxw8 with other F-box proteins which function to recruit substrates into the E3 ligase complex. The formation of this high order complex is likely to increase polyubiquitination efficiency.