Characterization of Mayven, a novel actin-binding protein predominantly expressed in brain

A novel zebrafish kelchlike gene klhl and its human ortholog KLHL display conserved expression patterns in skeletal and cardiac muscles

In this study, a novel gene, kelchlike (klhl) was identified in zebrafish by whole-mount in situ hybridization screen for important genes involved in embryogenesis. A full-length klhl cDNA was cloned and characterized. We found that klhl was a member of the kelch-repeat superfamily, containing two evolutionary conserved domains--broad-complex, tramtrack, bric-a-brac/poxvirus and zinc finger (BTB/POZ) domain, and kelch motif. Database mining revealed the presence of putative orthologs of klhl in human, mouse, rat, and pufferfish. klhl was determined to map to zebrafish linkage group (LG) 13 and was found to be syntenic with the proposed orthologs of klhl in human, mouse, and rat. In an effort to elucidate the function of klhl, klhl expression was investigated by Northern blot analysis and in situ hybridization. klhl is specifically expressed in the fast skeletal and cardiac muscle. Northern blot analyses show that the human ortholog, KLHL, is also specifically expressed in the skeletal muscles and heart. In silico analyses of rat expressed sequence tag (EST) clones corresponding to rat Klhl ortholog also indicate that its expression is also restricted to rat muscle tissues, suggesting a conserved role of klhl in vertebrates. The expression pattern of klhl, as well as the presence of the kelch repeats indicates a possible role for Klhl in the organization of striated muscle cytoarchitecture.

Characterization of a novel splicing variant of KLHL5, a member of the kelch protein family

The kelch-repeat protein family is a recently found new kind of actin-binding protein. It is characterized by tandemly arranged motifs of about 50 amino acids. Previous study showed that most members of the kelch-repeat family were cytoskeletal proteins implicated in various cellular processes, such as actin cytoskeleton interaction, cytoplasmic sequestration of transcription factors and cell morphology. And some of the family members play important roles in tissue development, such as human ENC-1, NRP/B, etc. Another characteristic of the kelch family is that most members have another conserved BTB domain at the extreme amino terminus. The BTB domain is also found at the N-terminus of 5-10% of zinc-finger transcription factor types and is a conserved protein-protein interaction motif. During the large-scale sequencing analysis of a human fetal brain cDNA library we found a novel kelch-like protein gene 5, KLHL5, KLHL5 has high identity with Drosophila kelch protein and many other family members. Here we report a novel splicing variants of KLHL5, named KLHL5b and the expression pattern of KLHL5b in many tissues.

Molecular analysis of muskelin identifies a conserved discoidin-like domain that contributes to protein self-association

Muskelin is an intracellular protein with a C-terminal kelch-repeat domain that was initially characterized as having functional involvement in cell spreading on the extracellular matrix glycoprotein thrombospondin-1. As one approach to understanding the functional properties of muskelin, we have combined bioinformatic and biochemical studies. Through analysis of a new dataset of eight animal muskelins, we showed that the N-terminal region of the polypeptide corresponds to a predicted discoidin-like domain. This domain architecture is conserved in fungal muskelins and reveals a structural parallel between the muskelins and certain extracellular fungal galactose oxidases, although the phylogeny of the two groups appears distinct. In view of the fact that a number of kelch-repeat proteins have been shown to self-associate, co-immunoprecipitation, protein pull-down assays and studies of cellular localization were carried out with wild-type, deletion mutant and point mutant muskelins to investigate the roles of the discoidin-like and kelch-repeat domains. We obtained evidence for cis- and trans-interactions between the two domains. These studies provide evidence that muskelin self-associates through a head-to-tail mechanism involving the discoidin-like domain.

Botulinum Toxin Type A Targets RhoB to Inhibit Lysophosphatidic Acid-Stimulated Actin Reorganization and Acetylcholine Release in Nerve Growth Factor-Treated PC12 Cells

Botulinum toxin type A (BoNT/A) produced by Clostridium botulinum inhibits Ca2+-dependent acetylcholine (ACh) release (neuroexocytosis) at peripheral neuromuscular junctions, sometimes causing neuromuscular paralysis. This inhibitory effect is attributed to its metalloprotease activity to cleave the 25-kDa synaptosomal-associated protein, which is essential for the exocytotic machinery. However, deletion of this protein does not result in a complete block of neuroexocytosis, suggesting that botulinum-mediated inhibition may occur via another mechanism. Rho GTPases, a class of small GTP-binding proteins (G proteins), control actin cytoskeletal organization, thereby regulating a variety of cellular functions in various cells, including neuronal cells. We have shown that the G protein activator lysophosphatidic acid (LPA) triggered actin reorganization followed by Ca2+-dependent ACh release in nerve growth factor-treated PC12 cells and that BoNT/A blocked both events through degradation of RhoB by the proteasome. Overexpression of wild-type RhoB caused actin reorganization and enhanced the release of ACh by LPA to overcome toxin's inhibitory effect on actin reorganization/exocytosis stimulated by LPA, whereas overexpression of a dominant negative RhoB inhibited ACh release, regardless of LPA and/or toxin treatment. Finally, a knockdown of the RhoB gene via sequence-specific, post-transcriptional gene silencing reduced RhoB expression in PC12 cells, resulting in total inhibition of both actin reorganization and ACh release induced by LPA. We conclude that the RhoB signaling pathway regulates ACh release via actin cytoskeletal reorganization and that botulinum toxin inhibits neuroexocytosis by targeting RhoB pathway.

Haploinsufficiency of kelch-like protein homolog 10 causes infertility in male mice

We identified a testis-specific gene encoding a protein containing a BTB/POZ domain and six kelch repeats, which we named kelch homolog 10 (KLHL10). KLHL10 displays high evolutionary conservation in mammals, as evidenced by 98.7% amino acid identity between mouse and human KLHL10. KLHL10 is exclusively expressed in the cytoplasm of elongating and elongated spermatids (steps 9-16). We generated a Klhl10 null allele in 129S6/SvEv mouse embryonic stem cells, and obtained 47 chimeras from six independent embryonic stem cell lines. Whereas low-percentage male chimeras only produce C57BL/6J offspring, high-percentage chimeric and heterozygous males were completely infertile because of disrupted spermiogenesis characterized by asynchronous spermatid maturation, degeneration of late spermatids, sloughing of postmeiotic germ cells from the seminiferous epithelium, and marked reduction in the numbers of late spermatids. Our data demonstrate that, like protamine-1 and -2, both alleles of Klhl10 are required for male fertility and that haploinsufficiency caused by a mutation in one allele of Klhl10 prevents genetic transmission of both mutant and WT alleles.

Neuromuscular expression of the BTB/POZ and zinc finger protein myoneurin

Myoneurin belongs to the BTB/POZ and zinc finger protein family whose members have been implicated in regulatory functions of gene expression. Myoneurin has been identified in various tissues, but muscle is a privileged site of myoneurin gene transcription. We examined the neuromuscular expression of myoneurin during development and after axotomy. Myoneurin expression is developmentally regulated in mouse muscle and appeared to be associated with neuromuscular junctions during the late embryonic period. Myoneurin is located in and around synaptic myonuclei in mouse and human adult muscle. The expression of myoneurin is dysregulated after nerve section. Thus, the restricted myoneurin expression in synaptic myonuclei appeared to be controlled by muscle electrical activity. Myoneurin is identified within the peripheral condensed chromatin and the euchromatin/heterochromatin regions, and thus fulfills structural and expression criteria to represent a synaptic gene regulator.

Scaffolding of Keap1 to the actin cytoskeleton controls the function of Nrf2 as key regulator of cytoprotective phase 2 genes

Transcription factor Nrf2 regulates basal and inducible expression of phase 2 proteins that protect animal cells against the toxic effects of electrophiles and oxidants. Under basal conditions, Nrf2 is sequestered in the cytoplasm by Keap1, a multidomain, cysteinerich protein that is bound to the actin cytoskeleton. Keap1 acts both as a repressor of the Nrf2 transactivation and as a sensor of phase 2 inducers. Electrophiles and oxidants disrupt the Keap1-Nrf2 complex, resulting in nuclear accumulation of Nrf2, where it enhances the transcription of phase 2 genes via a common upstream regulatory element, the antioxidant response element. Reporter cotransfection-transactivation analyses with a series of Keap1 deletion mutants revealed that in the absence of the double glycine repeat domain Keap1 does not bind to Nrf2. In addition, deletion of either the intervening region or the C-terminal region also abolished the ability of Keap1 to sequester Nrf2, indicating that all of these domains contribute to the repressor activity of Keap1. Immunocytochemical and immunoprecipitation analyses demonstrated that Keap1 associates with actin filaments in the cytoplasm through its double glycine repeat domain. Importantly, disruption of the actin cytoskeleton promotes nuclear entry of an Nrf2 reporter protein. The actin cytoskeleton therefore provides scaffolding that is essential for the function of Keap1, which is the sensor for oxidative and electrophilic stress.

Novel kelch-like protein, KLEIP, is involved in actin assembly at cell-cell contact sites of Madin-Darby canine kidney cells

Dynamic rearrangements of cell-cell adhesion underlie a diverse range of physiological processes, but their precise molecular mechanisms are still obscure. Thus, identification of novel players that are involved in cell-cell adhesion would be important. We isolated a human kelch-related protein, Kelch-like ECT2 interacting protein (KLEIP), which contains the broad-complex, tramtrack, bric-a-brac (BTB)/poxvirus, zinc finger (POZ) motif and six-tandem kelch repeats. KLEIP interacted with F-actin and was concentrated at cell-cell contact sites of Madin-Darby canine kidney cells, where it colocalized with F-actin. Interestingly, this localization took place transiently during the induction of cell-cell contact and was not seen at mature junctions. KLEIP recruitment and actin assembly were induced around E-cadherin-coated beads placed on cell surfaces. The actin depolymerizing agent cytochalasin B inhibited this KLEIP recruitment around E-cadherin-coated beads. Moreover, constitutively active Rac1 enhanced the recruitment of KLEIP as well as F-actin to the adhesion sites. These observations strongly suggest that KLEIP is localized on actin filaments at the contact sites. We also found that N-terminal half of KLEIP, which lacks the actin-binding site and contains the sufficient sequence for the localization at the cell-cell contact sites, inhibited constitutively active Rac1-induced actin assembly at the contact sites. We propose that KLEIP is involved in Rac1-induced actin organization during cell-cell contact in Madin-Darby canine kidney cells.

Molecular phylogeny of the kelch-repeat superfamily reveals an expansion of BTB/kelch proteins in animals

Background

The kelch motif is an ancient and evolutionarily-widespread sequence motif of 44–56 amino acids in length. It occurs as five to seven repeats that form a β-propeller tertiary structure. Over 28 kelch-repeat proteins have been sequenced and functionally characterised from diverse organisms spanning from viruses, plants and fungi to mammals and it is evident from expressed sequence tag, domain and genome databases that many additional hypothetical proteins contain kelch-repeats. In general, kelch-repeat β-propellers are involved in protein-protein interactions, however the modest sequence identity between kelch motifs, the diversity of domain architectures, and the partial information on this protein family in any single species, all present difficulties to developing a coherent view of the kelch-repeat domain and the kelch-repeat protein superfamily. To understand the complexity of this superfamily of proteins, we have analysed by bioinformatics the complement of kelch-repeat proteins encoded in the human genome and have made comparisons to the kelch-repeat proteins encoded in other sequenced genomes.

Results

We identified 71 kelch-repeat proteins encoded in the human genome, whereas 5 or 8 members were identified in yeasts and around 18 in C. elegans, D. melanogaster and A. gambiae. Multiple domain architectures were identified in each organism, including previously unrecognised forms. The vast majority of kelch-repeat domains are predicted to form six-bladed β-propellers. The most prevalent domain architecture in the metazoan animal genomes studied was the BTB/kelch domain organisation and we uncovered 3 subgroups of human BTB/kelch proteins. Sequence analysis of the kelch-repeat domains of the most robustly-related subgroups identified differences in β-propeller organisation that could provide direction for experimental study of protein-binding characteristics.

Conclusion

The kelch-repeat superfamily constitutes a distinct and evolutionarily-widespread family of β-propeller domain-containing proteins. Expansion of the family during the evolution of multicellular animals is mainly accounted for by a major expansion of the BTB/kelch domain architecture. BTB/kelch proteins constitute 72 % of the kelch-repeat superfamily of H. sapiens and form three subgroups, one of which appears the most-conserved during evolution. Distinctions in propeller blade organisation between subgroups 1 and 2 were identified that could provide new direction for biochemical and functional studies of novel kelch-repeat proteins.

The BTB protein MEL-26 is a substrate-specific adaptor of the CUL-3 ubiquitin-ligase

Many biological processes, such as development and cell cycle progression are tightly controlled by selective ubiquitin-dependent degradation of key substrates. In this pathway, the E3-ligase recognizes the substrate and targets it for degradation by the 26S proteasome. The SCF (Skp1-Cul1-F-box) and ECS (Elongin C-Cul2-SOCS box) complexes are two well-defined cullin-based E3-ligases. The cullin subunits serve a scaffolding function and interact through their C terminus with the RING-finger-containing protein Hrt1/Roc1/Rbx1, and through their N terminus with Skp1 or Elongin C, respectively. In Caenorhabditis elegans, the ubiquitin-ligase activity of the CUL-3 complex is required for degradation of the microtubule-severing protein MEI-1/katanin at the meiosis-to-mitosis transition. However, the molecular composition of this cullin-based E3-ligase is not known. Here we identified the BTB-containing protein MEL-26 as a component required for degradation of MEI-1 in vivo. Importantly, MEL-26 specifically interacts with CUL-3 and MEI-1 in vivo and in vitro, and displays properties of a substrate-specific adaptor. Our results suggest that BTB-containing proteins may generally function as substrate-specific adaptors in Cul3-based E3-ubiquitin ligases.

Specific over-expression of deltex and a new Kelch-like protein in human germinal center B cells

Ig gene hypermutation was originally described as the molecular process underlying B cell affinity maturation following a T-dependent immune response. Somatic hypermutation is also used in some species such as sheep, to generate diversity during formation of the primary antibody repertoire. In sheep, B cells mutate their Ig receptor during antigen-independent development in the lymphoid follicles of ileal Peyer's patches, but this process is arrested when these same B cells are cultured in vitro. We have used these differences between in vivo and in vitro B cell development to perform a cDNA subtraction between these two cell populations, in order to search for genes that might be involved in the hypermutation process. We describe in this paper the characterization of two genes, highly expressed in sheep ileal Peyer's patch B cells and also in centroblasts of human tonsils: deltex (Drosophila) homolog 1 (DTX1), which is related to the Notch pathway and a new Kelch-like protein, KLHL6. The putative role of these proteins, which are more likely involved in the germinal center B cell differentiation pathway than in the hypermutation mechanism per se, is discussed.

Identification of bladder cancer antigens recognized by IgG antibodies of a patient with metastatic bladder cancer

To identify tumor antigens useful for the diagnosis and treatment of patients with bladder cancer, a lambda phage cDNA library constructed from a high-grade bladder cancer cell line was screened with autologous serum from a patient with metastatic bladder cancer. Forty-eight distinct antigens were isolated. By evaluating the immunogenicity and the tissue-specific expression, KU-BL-1 and KU-BL-2 were identified as immunogenic antigens with restricted tissue expression. KU-BL-1 was found to be a putative human lipoic acid synthetase with a metal-binding site, CXXXCXXC, that was expressed in bladder cancer cell lines and most bladder cancer tissues, as well as normal bladder mucosa and testis tissues. Immunoglobulin (Ig)G antibody to KU-BL-1 was detected in 2 of 28 patients with bladder cancer, but not in 30 healthy individuals. KU-BL-2 was found to be a putative human kelch-like protein that was homologous to Drosophila kelch, with a BTB/POZ domain and kelch repeats. KU-BL-2 was expressed in bladder cancer cell lines, most bladder cancer tissues, testis and heart, but not in normal bladder mucosa. IgG antibody to KU-BL-2 was detected in 8 of 28 patients with bladder cancer, but not in 16 healthy individuals. Tumor reactive T cells were induced from peripheral blood mononuclear cells (PBMC) by stimulation with one of the HLA-A24 binding KU-BL-2 peptides. Therefore, KU-BL-1 and KU-BL-2, which showed preferential expression in bladder cancer with restricted expression in normal tissues, as well as immunogenicity in multiple patients with bladder cancer, may be useful for the development of diagnostic and therapeutic methods for patients with bladder cancer.

Identification of Nd1, a novel murine kelch family protein, involved in stabilization of actin filaments

We isolated Nd1, a novel kelch family gene that encodes two forms of proteins, Nd1-L and Nd1-S. Nd1-L contains a BTB/POZ domain in its N terminus and six kelch repeats in the C terminus. Nd1-S has the BTB/POZ domain but lacks the six kelch repeats. Nd1-L but not Nd1-S mRNA is detected ubiquitously in normal mouse tissues. Nd1-L and Nd1-S proteins can form a dimer through the BTB/POZ domain. Nd1-L colocalizes with actin filaments detected using a confocal microscope, and its kelch repeats bind to them in vitro. Overexpression of Nd1-L in NIH3T3 cells delayed cell growth by affecting the transition of cytokinesis. Furthermore, the overexpression prevented NIH3T3 cells from cell death induced by actin destabilization but not by microtubule dysfunction. These data suggest that Nd1-L functions as a stabilizer of actin filaments as an actin-binding protein and may play a role in the dynamic organization of the actin cytoskeleton.

The Keap1 BTB/POZ dimerization function is required to sequester Nrf2 in cytoplasm

Transactivation of phase II detoxification enzymes and antioxidant proteins is mediated by the Cap'N'Collar transcription factor, Nrf2, which is sequestered in the cytoplasm by the actin-binding protein Keap1. Mutation of a conserved serine (S104A) within the Keap1 BTB/POZ domain disrupts Keap1 dimerization and eliminates the ability of Keap1 to sequester Nrf2 in the cytoplasm and repress Nrf2 transactivation. Disruption of endogenous Keap1 dimerization using BTB/POZ dominant negative proteins also inhibits the ability of Keap1 to retain Nrf2 in the cytoplasm. Exposure to an electrophilic agent that induces Nrf2 release and nuclear translocation disrupts formation of a Keap1 complex in vivo. Collectively, these data support the conclusion that Keap1 dimerization is required for Nrf2 sequestration and transcriptional repression. Furthermore, exposure to inducing agents disrupts the Keap1 dimerization function and results in Nrf2 release.

Characterization of a Drosophila melanogaster orthologue of muskelin

Muskelin was identified in vertebrates as a novel, intracellular, kelch repeat protein that is needed in cell-spreading responses to the matrix adhesion molecule, thrombospondin-1. The identification and characterization of an orthologue of muskelin in Drosophila melanogaster is now reported. The Drosophila muskelin gene, located on chromosome 2R, is encoded in ten exons. Drosophila muskelin is expressed in embryos, larvae and adult flies. The protein has 45% sequence identity to vertebrate muskelins, with highest sequence identity in an amino-terminal domain and the six kelch repeats that form a beta-propeller structure. Multiple sequence alignment of human, mouse, rat and Drosophila muskelins and protein database searches revealed a novel highly conserved motif within the amino-terminal domain, lissencephaly homology motif (LisH) and C-terminal to LisH motifs in the central region of the molecule, and several conserved consensus motifs for phosphorylation by protein kinase C and casein kinase II. These findings provide new information on the modular structure of muskelin and indicate potential for conserved mechanisms of function.

Actinfilin, a brain-specific actin-binding protein in postsynaptic density

The dynamic assembly and disassembly of actin-based cytoskeleton is closely linked to the changes in the postsynaptic density in both number and shape, which is thought to be important in forming long-term memory. Thus, regulation of actin filaments may play a critical role in contributing to the formation of long-term memory. Here, we report the cloning of actinfilin, a brain-specific Kelch protein, which interacts with F-actin. Actinfilin contains an amino-terminal POZ/BTB domain and carboxyl positioned six tandem Kelch repeats that presumably form six blades of beta-propeller structure of the Kelch domain. Co-immunoprecipitation analyses showed that the amino-terminal POZ domain mediated actinfilin-actinfilin interaction. The recombinant Kelch domain alone was sufficient to mediate binding to F-actin. Immunohistochemistry studies of rat brain sections suggested that actinfilin is broadly expressed in neurons of most regions of the brain. The subcellular localization of actinfilin was studied by biochemical fractionation and immunogold labeling. The results showed the postsynaptic density distribution of actinfilin. Together, these results indicate that actinfilin may be a key player in the actin-based neuronal function.

Selected BTB/POZ-kelch proteins bind ATP

Proteins with a bric-à-brac, tramtrack, broad-complex/Poxvirus zinc fingers (BTB/POZ) domain are implicated in a broad variety of biological processes, including DNA binding, regulation of gene transcription and organization of macromolecular structures. Kelch domain containing BTB/POZ proteins like Mayven and Keap1 display limited sequence similarity with the actin-fragmin kinase from Physarum, a protein kinase with a kelch domain. We show that mouse Keap1, a Caenorhabditis elegans protein that we named CKR, and human Mayven bind 5'-p-fluorosulfonyl-benzoyl-adenosine (FSBA), a covalently modifying ATP analogue. Binding with 2-azido-ATP or ATP-Sepharose is also demonstrated. In contrast to Mayven, FSBA binding by CKR and Keap1 was specifically inhibited by excess ATP. The ATP binding pocket is located in the N-terminal half of Keap1. Our findings indicate that several, but not all, BTB/POZ-kelch domain proteins possess an inconspicuous ATP binding cassette.

The KLHL1-antisense transcript ( KLHL1AS) is evolutionarily conserved

Spinocerebellar ataxia type 8 (SCA8) is caused by a CTG expansion in an untranslated, endogenous antisense RNA that overlaps the Kelch-like 1 ( KLHL1) gene. The normal function of this transcript is currently unknown. We have now identified the promoter region for the KLHL1-antisense ( KLHL1AS) RNA and report that a Klhl1as transcript is present in the mouse as well. Human and mouse KLHL1AS are transcribed from homologous promoter regions in the first intron of KLHL1 and extend through the transcription and translation start sites as well as the first splice donor sequence of KLHL1. We found that the mouse Klhl1as RNA is not spliced and terminates in a polyadenylation site in the Klhl1 promoter region, whereas both the present and previous work show that human KLHL1AS is highly variably spliced into processed transcripts that contain up to six exons. Mouse Klhl1as transcript was detected in RNA isolated from the cerebellum and from total adult brain and total fetal tissue, and at a low level in testis and ovary. Similarly, human KLHL1AS is expressed in various brain tissues, including the cerebellum, the tissue most affected by SCA8, and was detected at low levels in testis and kidney. The evolutionary conservation of this antisense/sense transcriptional organization strongly indicates that KLHL1AS transcripts play a significant biological role in both human and mouse, presumably as a regulator of KLHL1 expression.

Drosophila Kelch regulates actin organization via Src64-dependent tyrosine phosphorylation

The Drosophila kelch gene encodes a member of a protein superfamily defined by the presence of kelch repeats. In Drosophila, Kelch is required to maintain actin organization in ovarian ring canals. We set out to study the actin cross-linking activity of Kelch and how Kelch function is regulated. Biochemical studies using purified, recombinant Kelch protein showed that full-length Kelch bundles actin filaments, and kelch repeat 5 contains the actin binding site. Two-dimensional electrophoresis demonstrated that Kelch is tyrosine phosphorylated in a src64-dependent pathway. Site-directed mutagenesis determined that tyrosine residue 627 is phosphorylated. A Kelch mutant with tyrosine 627 changed to alanine (KelY627A) rescued the actin disorganization phenotype of kelch mutant ring canals, but failed to produce wild-type ring canals. Electron microscopy demonstrated that phosphorylation of Kelch is critical for the proper morphogenesis of actin during ring canal growth, and presence of the nonphosphorylatable KelY627A protein phenocopied src64 ring canals. KelY627A protein in ring canals also dramatically reduced the rate of actin monomer exchange. The phenotypes caused by src64 mutants and KelY627A expression suggest that a major function of Src64 signaling in the ring canal is the negative regulation of actin cross-linking by Kelch.

Cloning and characterization of ectopically expressed transcripts for the actin-binding protein MIPP in mouse mammary carcinomas

Mipp is a kelch-related, placental-specific gene that is ectopically expressed in many BALB/c mouse mammary carcinomas of various etiologies. The Kelch family encompasses proteins that are emerging as key links between microfilaments and a variety of cellular structures and functions. Mouse mammary tumors express two mipp transcripts (2.2 and 5.6 kb). We cloned the 2.2 kb mipp mRNA and analysed the product of its 1.7 kb ORF. The 584 residue MIPP protein has an N-terminal BTB domain and six C-terminal tandem Kelch repeats. Despite expression of two mipp RNAs, only a single MIPP protein is expressed in mammary tumors. MIPP protein binds to microfilaments in vitro and co-immunoprecipitates with actin. MIPP co-localized with concanavalin A at the endoplasmic reticulum, suggesting that MIPP might mediate interactions between microtubules and actin filaments. Because MIPP expression is widespread in mouse mammary tumors, it might contribute to tumorigenesis. Although MIPP had little effect on the growth rate of human breast cell lines following transfection, it greatly reduced the formation of duct-like structures on reconstituted basement membrane. Our results suggest that MIPP could contribute to malignant progression in the mouse mammary epithelial cells by perverting their response to cues from the extracellular matrix.

Transcript map and comparative analysis of the 1.5-Mb commonly deleted segment of human 5q31 in malignant myeloid diseases with a del(5q)

Loss of a whole chromosome 5, or a del(5q), are recurring abnormalities in malignant myeloid diseases. In previous studies, we defined a commonly deleted segment (CDS) of 1.5 Mb between D5S479 and D5S500 in patients with a del(5q), and we established a P1 artificial chromosome-based contig encompassing this interval. To identify candidate tumor suppressor genes (TSGs), we developed a transcript map of the CDS. The map contains 18 genes and 12 expressed sequence tags/UniGenes. Among the 18 genes are 10 genes that were previously cloned and 8 novel genes. The newly identified genes include CDC23, which encodes a component of the anaphase-promoting complex; RAB6KIFL, which encodes a kinesin-like protein involved in organelle transport; and KLHL3, which encodes a human homologue of the Drosophila ring canal protein, kelch. We determined the intron/exon organization of 14 genes and eliminated each gene as a classical TSG by mutation analysis. In addition, we established a single-nucleotide polymorphism map as well as a map of the mouse genome that is syntenic to the CDS of human 5q31. The development of a transcription map will facilitate the molecular cloning of a myeloid leukemia suppressor gene on 5q.

The gene encoding gigaxonin, a new member of the cytoskeletal BTB/kelch repeat family, is mutated in giant axonal neuropathy

Disorganization of the neurofilament network is a prominent feature of several neurodegenerative disorders including amyotrophic lateral sclerosis (ALS), infantile spinal muscular atrophy and axonal Charcot-Marie-Tooth disease. Giant axonal neuropathy (GAN, MIM 256850), a severe, autosomal recessive sensorimotor neuropathy affecting both the peripheral nerves and the central nervous system, is characterized by neurofilament accumulation, leading to segmental distension of the axons. GAN corresponds to a generalized disorganization of the cytoskeletal intermediate filaments (IFs), to which neurofilaments belong, as abnormal aggregation of multiple tissue-specific IFs has been reported: vimentin in endothelial cells, Schwann cells and cultured skin fibroblasts, and glial fibrillary acidic protein (GFAP) in astrocytes. Keratin IFs also seem to be alterated, as most patients present characteristic curly or kinky hairs. We report here identification of the gene GAN, which encodes a novel, ubiquitously expressed protein we have named gigaxonin. We found one frameshift, four nonsense and nine missense mutations in GAN of GAN patients. Gigaxonin is composed of an amino-terminal BTB (for Broad-Complex, Tramtrack and Bric a brac) domain followed by a six kelch repeats, which are predicted to adopt a beta-propeller shape. Distantly related proteins sharing a similar domain organization have various functions associated with the cytoskeleton, predicting that gigaxonin is a novel and distinct cytoskeletal protein that may represent a general pathological target for other neurodegenerative disorders with alterations in the neurofilament network.

Molecular cloning and characterization of a novel human gene containing ankyrin repeat and double BTB/POZ domain

A novel human gene containing an ankyrin repeat and BTB/POZ domains (BPOZ) was isolated from a human leukocyte cDNA library. The cDNA sequence contains an open reading frame of 1434 bp that encodes 478 amino acid residues with a predicted molecular mass of 53.9 kDa. Sequence pattern analysis shows that BPOZ contains an N-terminal ankyrin repeat, a bipartite nuclear localization signal and two BTB/POZ domains. Using semiquantitative RT-PCR, the BPOZ transcript was found to be ubiquitously expressed in all fetal tissues examined (heart, brain, liver, and kidney) suggesting that BPOZ is involved in basic cellular function. Low expression of BPOZ in adult tissues (normal and hypertrophic heart) suggests that BPOZ mRNA is developmentally regulated and may play a role in developmental processes. Chromosomal localization by radiation hybrid mapping revealed that this gene is localized between D3S1269 and D3S3606 markers corresponding to the region of chromosome 3q21, a region frequently associated with leukemia. It is thus suggested that BPOZ may be functionally involved in protein-protein interaction, perhaps in forming protein complexes, and may have an important role in normal development and in the development of leukemia.

Transient induction of ENC-1, a Kelch-related actin-binding protein, is required for adipocyte differentiation

In an attempt to study molecules that play a regulatory role early in adipocyte differentiation, we identified by differential display ENC-1, a Drosophila kelch-related protein. ENC-1 colocalizes with actin filaments. ENC-1 is expressed in adipose tissue, specifically in the adipose-derived stroma-vascular fraction. ENC-1 mRNA levels are transiently increased 8-12-fold early in in vitro adipocyte differentiation of primary cells of the adipose-derived stroma-vascular fraction and of 3T3-L1 cells. Treatment with the adipogenic inducers dexamethasone and methylisobutylxanthine causes an increase in ENC-1 mRNA levels specifically in preadipocytes, and methylisobutylxanthine is the main effector of ENC-1 expression. The induction of ENC-1 precedes expression of the transcription factors, peroxisome proliferator-activated receptor (PPARgamma) and CCAAT/enhancer-binding protein (C/EBPalpha), and other adipocyte markers including adipocyte fatty acid-binding protein. The ENC-1 induction correlates with the subsequent differentiation of primary stroma-vascular cells into adipocytes. Furthermore, decreasing the endogenous ENC-1 levels by stable antisense transfection, thereby preventing the transient induction, effectively inhibits 3T3-L1 adipocyte differentiation. Overall, these studies indicate that ENC-1, an actin-binding protein, plays a regulatory role early in adipocyte differentiation when cytoskeletal reorganization and cell shape change from fibroblastic preadipocytes to spherical adipocytes occur.

Molecular characterization of KLHL3, a human homologue of the Drosophila kelch gene

The Drosophila kelch protein is a structural component of ring canals and is required for oocyte maturation. Here, we report the cloning and genomic structure of a new human homologue of kelch, KLHL3. At the amino acid level, KLHL3 shares 77% similarity with Drosophila kelch and 89% similarity with Mayven (KLHL2), another human kelch homolog. The approximately 6.5-kb mRNA has a single open reading frame encoding a protein of 587 amino acids with a predicted molecular mass of 650 kDa. Like kelch and KLHL2, the KLHL3 protein contains a poxvirus and zinc finger domain at the N-terminus and six tandem repeats (kelch repeats) at the C-terminus. At least three isoforms, which differ in the length of the N-terminus, are produced and may be the result of alternative promoter usage. We also identified alternative polyadenylation sites and alternative splicing; thus, as many as 12 mRNA variants and six putative protein isoforms could be produced. The KLHL3 gene is mapped to human chromosome 5, band q31, contains 17 exons, and spans approximately 120 kb of genomic DNA. KLHL3 maps within the smallest commonly deleted segment in myeloid leukemias characterized by a deletion of 5q; however, we detected no inactivating mutations of KLHL3 in malignant myeloid disorders with loss of 5q.

Krp1, a novel kelch related protein that is involved in pseudopod elongation in transformed cells

We have previously shown that the transcription factor AP-1 regulates the expression of genes which allow neoplastically transformed rat fibroblasts to become invasive. Searches for further AP-1 target genes led to the identification of a gene encoding a novel rat kelch family member, named kelch related protein 1 (Krp1). Kelch family members are characterized by a series of repeats at their carboxyl terminus and a BTB/POZ domain near their amino terminus. Rat Krp1 has a primarily cytoplasmic localization, and a small fraction appears to accumulate and co-localize with F-actin at membrane ruffle-like structures in the tips of pseudopodia. Overexpression of Krp1 in transformed rat fibroblasts led to the formation of dramatically elongated pseudopodia, while expression of truncated Krp1 polypeptides resulted in a reduction in the length of pseudopodia. We propose that the transformation-specific expression of Krp1 is required for pseudopod elongation, which are structures that are required for cell motility and invasion.

The kelch repeat superfamily of proteins: propellers of cell function

The kelch motif was discovered as a sixfold tandem element in the sequence of the Drosophila kelch ORF1 protein. The repeated kelch motifs predict a conserved tertiary structure, a beta-propeller. This module appears in many different polypeptide contexts and contains multiple potential protein-protein contact sites. Members of this growing superfamily are present throughout the cell and extracellularly and have diverse activities. In this review, we discuss current information concerning the structural organization of kelch repeat proteins, their biological roles and the molecular basis of their action.