HIP14, a novel ankyrin domain-containing protein, links huntingtin to intracellular trafficking and endocytosis

Huntington disease (HD) is caused by polyglutamine [poly(Q)] expansion in the protein huntingtin (htt). Although the exact mechanism of disease progression remains to be elucidated, altered interactions of mutant htt with its protein partners could contribute to the disease. Using the yeast two-hybrid system, we have isolated a novel htt interacting protein, HIP14. HIP14's interaction with htt is inversely correlated to the poly(Q) length in htt. mRNAs of 9 and 6 bp are transcribed from the HIP14 gene, with the 6 kb transcript being predominantly expressed in the brain. HIP14 protein is enriched in the brain, shows partial co-localization with htt in the striatum, and is found in medium spiny projection neurons, the subset of neurons affected in HD. HIP14 localizes to the Golgi, and to vesicles in the cytoplasm. The HIP14 protein has sequence similarity to Akr1p, a protein essential for endocytosis in Saccharomyces cerevisiae. Expression of human HIP14 results in rescue of the temperature-sensitive lethality in akr1 Delta yeast cells and, furthermore, restores their defect in endocytosis, demonstrating a role for HIP14 in intracellular trafficking. Our findings suggest that decreased interaction between htt and HIP14 could contribute to the neuronal dysfunction in HD by perturbing normal intracellular transport pathways in neurons.

Protein kinase A site-specific phosphorylation regulates ATP-binding cassette A1 (ABCA1)-mediated phospholipid efflux

ATP-binding cassette A1 (ABCA1) is a key mediator of cholesterol and phospholipid efflux to apolipoprotein particles. We show that ABCA1 is a constitutively phosphorylated protein in both RAW macrophages and in a human embryonic kidney cell line expressing ABCA1. Furthermore, we demonstrate that phosphorylation of ABCA1 is mediated by protein kinase A (PKA) or a PKA-like kinase in vivo. Through site-directed mutagenesis studies of consensus PKA phosphorylation sites and in vitro PKA kinase assays, we show that Ser-1042 and Ser-2054, located in the nucleotide binding domains of ABCA1, are major phosphorylation sites for PKA. ApoA-I-dependent phospholipid efflux was decreased significantly by mutation of Ser-2054 alone and Ser-1042/Ser-2054 but was not significantly impaired with Ser-1042 alone. The mechanism by which ABCA1 phosphorylation affected ApoA-I-dependent phospholipid efflux did not involve either alterations in ApoA-I binding or changes in ABCA1 protein stability. These studies demonstrate a novel serine (Ser-2054) on the ABCA1 protein crucial for PKA phosphorylation and for regulation of ABCA1 transporter activity.

Caspase cleavage of mutant huntingtin precedes neurodegeneration in Huntington's disease

Huntington's disease (HD) results from polyglutamine expansion in huntingtin (htt), a protein with several consensus caspase cleavage sites. Despite the identification of htt fragments in the brain, it has not been shown conclusively that htt is cleaved by caspases in vivo. Furthermore, no study has addressed when htt cleavage occurs with respect to the onset of neurodegeneration. Using antibodies that detect only caspase-cleaved htt, we demonstrate that htt is cleaved in vivo specifically at the caspase consensus site at amino acid 552. We detect caspase-cleaved htt in control human brain as well as in HD brains with early grade neuropathology, including one homozygote. Cleaved htt is also seen in wild-type and HD transgenic mouse brains before the onset of neurodegeneration. These results suggest that caspase cleavage of htt may be a normal physiological event. However, in HD, cleavage of mutant htt would release N-terminal fragments with the potential for increased toxicity and accumulation caused by the presence of the expanded polyglutamine tract. Furthermore, htt fragments were detected most abundantly in cortical projection neurons, suggesting that accumulation of expanded htt fragments in these neurons may lead to corticostriatal dysfunction as an early event in the pathogenesis of HD.

Increased sensitivity to N-methyl-D-aspartate receptor-mediated excitotoxicity in a mouse model of Huntington's disease

Previous work suggests N-methyl-D-aspartate receptor (NMDAR) activation may be involved in degeneration of medium-sized spiny striatal neurons in Huntington's disease (HD). Here we show that these neurons are more vulnerable to NMDAR-mediated death in a YAC transgenic FVB/N mouse model of HD expressing full-length mutant huntingtin, compared with wild-type FVB/N mice. Excitotoxic death of these neurons was increased after intrastriatal injection of quinolinate in vivo, and after NMDA but not AMPA exposure in culture. NMDA-induced cell death was abolished by an NR2B subtype-specific antagonist. In contrast, NMDAR-mediated death of cerebellar granule neurons was not enhanced, consistent with cell-type and NMDAR subtype specificity. Moreover, increased NMDA-evoked current amplitude and caspase-3 activity were observed in transgenic striatal neurons. Our data support a role for NR2B-subtype NMDAR activation as a trigger for selective neuronal degeneration in HD.

ABCA1 mRNA and protein distribution patterns predict multiple different roles and levels of regulation

Mutations in ABCA1 cause the allelic disorders familial hypolipoproteinemia and Tangier Disease. To identify where ABCA1 was likely to have a functional role, we determined the cellular and tissue-specific patterns of murine ABCA1 expression. RT-PCR and Western blot analysis on dissected murine tissues demonstrated broad expression of ABCA1 mRNA and protein in many tissues with prominent protein expression in liver, testis, and adrenal tissue. In situ hybridization and immunohistochemistry experiments demonstrated specific patterns of ABCA1 expression at the cellular level, with hepatocytes, the epithelial lining of the digestive system and bladder, the proximal convoluted tubule of the kidney, and Purkinje and cortical pyramidal neurons containing abundant ABCA1 protein. Significant discordance between relative mRNA and protein expression patterns suggests the possibility of post-transcriptional regulation of ABCA1 expression in selected cells or tissues. We also show that ABCA1 protein levels are up-regulated specifically in the liver after exposure to an atherogenic diet for 7 days, supporting a major role for the liver in dietary modulation of HDL-C levels. Our observations show that ABCA1 is expressed in a pattern consistent with its role in HDL-C metabolism. Additionally, ABCA1 may have important functional roles in other cell types independent of HDL-C regulation.

Human ABCA1 BAC transgenic mice show increased high density lipoprotein cholesterol and ApoAI-dependent efflux stimulated by an internal promoter containing liver X receptor response elements in intron 1

By using BAC transgenic mice, we have shown that increased human ABCA1 protein expression results in a significant increase in cholesterol efflux in different tissues and marked elevation in high density lipoprotein (HDL)-cholesterol levels associated with increases in apoAI and apoAII. Three novel ABCA1 transcripts containing three different transcription initiation sites that utilize sequences in intron 1 have been identified. In BAC transgenic mice there is an increased expression of ABCA1 protein, but the distribution of the ABCA1 product in different cells remains similar to wild type mice. An internal promoter in human intron 1 containing liver X response elements is functional in vivo and directly contributes to regulation of the human ABCA1 gene in multiple tissues and to raised HDL cholesterol, apoAI, and apoAII levels. A highly significant relationship between raised protein levels, increased efflux, and level of HDL elevation is evident. These data provide proof of the principle that increased human ABCA1 efflux activity is associated with an increase in HDL levels in vivo.

Huntington disease: new insights on the role of huntingtin cleavage

Huntington Disease (HD) results from polyglutamine expansion within the N-terminus of huntingtin. We have produced yeast artificial chromosome (YAC) transgenic mice expressing normal (YAC18) and mutant (YAC46 and YAC72) human huntingtin in a developmentally appropriate and tissue-specific manner identical to the pattern of expression of endogenous huntingtin. YAC46 and YAC72 mice show early electrophysiological abnormalities indicating neuronal cytoplasmic dysfunction prior to developing nuclear inclusions or neurodegeneration. YAC72 mice display a hyperkinetic movement disorder by 7 months of age, and have evidence for selective and specific degeneration of medium spiny neurons in the lateral striatum by 12 months of age. A key molecular feature of pathology of these YAC72 mice is cleavage of huntingtin in the cytoplasm following by translocation of the resulting huntingtin N-terminal fragments into the nucleus of striatal neurons. Increasing nuclear localization of huntingtin N-terminal fragments within medium spiny neurons of the striatum occurs concomitantly with the onset of selective neurodegeneration. Because huntingtin is a caspase substrate and truncated huntingtin fragments are toxic in vitro, inhibiting caspase cleavage of huntingtin may be of potential therapeutic benefit in HD. We show that caspase inhibitors eliminate huntingtin cleavage in cells and protects them from an apoptotic stress. We also identify caspase-6 and caspase-3 cleavage sites in huntingtin and demonstrate that neuronal and non-neuronal cells expressing a caspase-resistant huntingtin with an expanded polyglutamine tract are less susceptible to apoptosis and aggregate formation. These results suggest that caspase cleavage of huntingtin may be a crucial step in aggregate formation and neurotoxicity in HD.

Mutant huntingtin enhances excitotoxic cell death

Evidence suggests overactivation of NMDA-type glutamate receptors (NMDARs) contributes to selective degeneration of medium-sized spiny striatal neurons in Huntington's disease (HD). Here we determined whether expression of huntingtin containing the polyglutamine expansion augments NMDAR-mediated excitotoxicity. HEK293 cells coexpressing mutant huntingtin (htt-138Q) and either NR1A/NR2A- or NR1A/NR2B-type NMDARs exposed to 1 mM NMDA showed a significant increase in excitotoxic cell death compared to controls (cells coexpressing htt-15Q or GFP), but the difference was larger for NR1A/NR2B. Moreover, agonist-dependent cell death showed apoptotic features for cells coexpressing htt-138Q and NR1A/NR2B, but not for cells expressing htt-138Q and NR1A/NR2A. Further, NR1A/NR2B-mediated apoptosis was not seen with coexpression of an N-terminal fragment of mutant htt. Since NR1A/NR2B is the predominant NMDAR subtype in neostriatal medium-sized spiny neurons, enhancement of NMDA-induced apoptotic death in NR1A/NR2B-expressing cells by full-length mutant htt may contribute to selective neurodegeneration in HD.

Huntingtin interacting protein 1 induces apoptosis via a novel caspase-dependent death effector domain

Huntington disease is a devastating neurodegenerative disease caused by the expansion of a polymorphic glutamine tract in huntingtin. The huntingtin interacting protein (HIP-1) was identified by its altered interaction with mutant huntingtin. However, the function of HIP-1 was not known. In this study, we identify HIP-1 as a proapoptotic protein. Overexpression of HIP-1 resulted in rapid caspase 3-dependent cell death. Bioinformatics analyses identified a novel domain in HIP-1 with homology to death effector domains (DEDs) present in proteins involved in apoptosis. Expression of the HIP-1 DED alone resulted in cell death indistinguishable from HIP-1, indicating that the DED is responsible for HIP-1 toxicity. Furthermore, substitution of a conserved hydrophobic phenylalanine residue within the HIP-1 DED at position 398 eliminated HIP-1 toxicity entirely. HIP-1 activity was found to be independent of the DED-containing caspase 8 but was significantly inhibited by the antiapoptotic protein Bcl-x(L), implicating the intrinsic pathway of apoptosis in HIP-1-induced cell death. Co-expression of a normal huntingtin fragment capable of binding HIP-1 significantly reduced cell death. Our data identify HIP-1 as a novel proapoptotic mediator and suggest that HIP-1 may be a molecular accomplice in the pathogenesis of Huntington disease.

Inhibiting caspase cleavage of huntingtin reduces toxicity and aggregate formation in neuronal and nonneuronal cells

Huntington's disease is a neurodegenerative disorder caused by CAG expansion that results in expansion of a polyglutamine tract at the extreme N terminus of huntingtin (htt). htt with polyglutamine expansion is proapoptotic in different cell types. Here, we show that caspase inhibitors diminish the toxicity of htt. Additionally, we define htt itself as an important caspase substrate by generating a site-directed htt mutant that is resistant to caspase-3 cleavage at positions 513 and 530 and to caspase-6 cleavage at position 586. In contrast to cleavable htt, caspase-resistant htt with an expanded polyglutamine tract has reduced toxicity in apoptotically stressed neuronal and nonneuronal cells and forms aggregates at a much reduced frequency. These results suggest that inhibiting caspase cleavage of htt may therefore be of potential therapeutic benefit in Huntington's disease.

Recent insights into the molecular pathogenesis of Huntington disease

Huntington disease (HD) is a neurodegenerative disorder caused by a CAG repeat expansion in the HD gene resulting in expression of an uninterrupted polyglutamine stretch within the N-terminus of its protein product huntingtin (htt). In this article we review the clinical, genetic, and neuropathological features of HD and discuss recent insights into the pathogenesis of HD. Examining the role of CAG repeat size on age of onset and penetrance in HD using a refined database of human HD patients has provided further support for the importance of the CAG repeat in the pathogenesis of HD and information leading to a predictive model for the likelihood of being affected by a specific age for a particular CAG expansion. In a YAC transgenic mouse model that replicates key elements of the HD phenotype, the development of selective striatal neurodegeneration is coincident with cleavage of htt and translocation of the N-terminal htt fragment into the nucleus. We also review in vitro evidence that htt is a substrate for cleavage by a group of cysteine proteases involved in apoptotic death-the caspases, and that caspase cleavage of htt results in the generation of a toxic N-terminal fragment. Inhibiting caspase cleavage of huntingtin eliminates the toxicity of the mutant htt protein. These results suggest that cleavage of huntingtin resulting in production of a truncated N-terminal fragment may be a crucial step in the pathogenesis of Huntington disease and that inhibition of this process may be a potential therapeutic strategy for this currently untreatable disorder.

Caspases and neurodegeneration: on the cutting edge of new therapeutic approaches

Unregulated apoptosis underlies many pathological conditions, including neurodegenerative diseases. In this review, we focus on the role of cysteine aspartate-specific proteases (caspase) activity in Huntington disease (HD) and Alzheimer disease (AD) as two representative neurodegenerative disorders that normally manifest in mid- to late-life. Caspases appear to be involved in the molecular pathology of HD by directly cleaving huntingtin and generating toxic protein fragments containing the polyglutamine tract, and by being recruited and activated by polyglutamine-containing aggregates composed mainly of truncated huntingtin fragments. Several proteins involved in AD, including beta-amyloid precursor protein (APP) and presenilins (PSs), are also cleaved by caspases. For APP, caspase cleavage may contribute to toxicity by generating toxic fragments or by shifting APP processing toward an amyloidogenic pathway. For PSs, caspase cleavage disables antiapoptotic functions attributed to PS C-terminal fragments. These observations suggest that caspases actively contribute to the molecular pathogenesis of these diseases and support the development of caspase inhibitors as potential therapeutic approaches for chronic neurodegenerative disorders.

Cleavage of atrophin-1 at caspase site aspartic acid 109 modulates cytotoxicity

Dentatorubropallidoluysian atrophy (DRPLA) is one of eight autosomal dominant neurodegenerative disorders characterized by an abnormal CAG repeat expansion which results in the expression of a protein with a polyglutamine stretch of excessive length. We have reported recently that four of the gene products (huntingtin, atrophin-1 (DRPLA), ataxin-3, and androgen receptor) associated with these open reading frame triplet repeat expansions are substrates for the cysteine protease cell death executioners, the caspases. This led us to hypothesize that caspase cleavage of these proteins may represent a common step in the pathogenesis of each of these four neurodegenerative diseases. Here we present evidence that caspase cleavage of atrophin-1 modulates cytotoxicity and aggregate formation. Cleavage of atrophin-1 at Asp109 by caspases is critical for cytotoxicity because a mutant atrophin-1 that is resistant to caspase cleavage is associated with significantly decreased toxicity. Further, the altered cellular localization within the nucleus and aggregate formation associated with the expanded form of atrophin-1 are completely suppressed by mutation of the caspase cleavage site at Asp109. These results provide support for the toxic fragment hypothesis whereby cleavage of atrophin-1 by caspases may be an important step in the pathogenesis of DRPLA. Therefore, inhibiting caspase cleavage of the polyglutamine-containing proteins may be a feasible therapeutic strategy to prevent cell death.

Kennedy's disease: caspase cleavage of the androgen receptor is a crucial event in cytotoxicity

X-linked spinal and bulbar muscular atrophy (SBMA), Kennedy's disease, is a degenerative disease of the motor neurons that is associated with an increase in the number of CAG repeats encoding a polyglutamine stretch within the androgen receptor (AR). Recent work has demonstrated that the gene products associated with open reading frame triplet repeat expansions may be substrates for the cysteine protease cell death executioners, the caspases. However, the role that caspase cleavage plays in the cytotoxicity associated with expression of the disease-associated alleles is unknown. Here, we report the first conclusive evidence that caspase cleavage is a critical step in cytotoxicity; the expression of the AR with an expanded polyglutamine stretch enhances its ability to induce apoptosis when compared with the normal AR. The AR is cleaved by a caspase-3 subfamily protease at Asp146, and this cleavage is increased during apoptosis. Cleavage of the AR at Asp146 is critical for the induction of apoptosis by AR, as mutation of the cleavage site blocks the ability of the AR to induce cell death. Further, mutation of the caspase cleavage site at Asp146 blocks the ability of the SBMA AR to form perinuclear aggregates. These studies define a fundamental role for caspase cleavage in the induction of neural cell death by proteins displaying expanded polyglutamine tracts, and therefore suggest a strategy that may be useful to treat neurodegenerative diseases associated with polyglutamine repeat expansions.

Quiescence versus apoptosis: Myc abundance determines pathway of exit from the cell cycle

When exposed to diverse growth conditions in vitro, cells can respond by entering states of proliferation, quiescence, differentiation or apoptosis. While the choices among these states can be influenced by proto-oncogene expression, how these disparate outcomes are achieved remains poorly understood. To address these issues, we have generated rodent fibroblast cell lines that harbor a human c-myc gene under the control of a tetracycline-regulated promoter. When Myc-induced cells are deprived of serum growth factors, they rapidly become apoptotic with the onset of apoptosis preceded by a large, transient increase in cdk2 kinase activity that is associated with the induction of cdc25A phosphatase and the later accumulation of p27Kip1 kinase inhibitor. Surprisingly, serum starvation in the absence of myc overexpression, (which leads to quiescence instead of apoptosis) also causes a marked transient elevation in cdk2 kinase activity, an induction of cdc25A and a delayed increase in p27Kip1. Transient elevations in cdk2 kinase activity and cdc25A abundance are required for cell cycle progression, but it is evident that these changes also precede entry to either apoptosis or quiescence in serum-starved cells. These findings suggest that the pathways to both quiescence and apoptosis share regulatory machinery with cell cycle control mechanisms. In addition, the abundance of Myc protein can be critical in the choices among these cellular states.

The influence of huntingtin protein size on nuclear localization and cellular toxicity

Huntington disease is an autosomal dominant neurodegenerative disorder caused by the pathological expansion of a polyglutamine tract. In this study we directly assess the influence of protein size on the formation and subcellular localization of huntingtin aggregates. We have created numerous deletion constructs expressing successively smaller fragments of huntingtin and show that these smaller proteins containing 128 glutamines form both intranuclear and perinuclear aggregates. In contrast, larger NH2-terminal fragments of huntingtin proteins with 128 glutamines form exclusively perinuclear aggregates. These aggregates can form in the absence of endogenous huntingtin. Furthermore, expression of mutant huntingtin results in increased susceptibility to apoptotic stress that is greater with decreasing protein length and increasing polyglutamine size. As both intranuclear and perinuclear aggregates are clearly associated with increased cellular toxicity, this supports an important role for toxic polyglutamine-containing fragments forming aggregates and playing a key role in the pathogenesis of Huntington disease.

Caspase cleavage of gene products associated with triplet expansion disorders generates truncated fragments containing the polyglutamine tract

The neurodegenerative diseases Huntington disease, dentatorubropallidoluysian atrophy, spinocerebellar atrophy type 3, and spinal bulbar muscular atrophy are caused by expansion of a polyglutamine tract within their respective gene products. There is increasing evidence that generation of truncated proteins containing an expanded polyglutamine tract may be a key step in the pathogenesis of these disorders. We now report that, similar to huntingtin, atrophin-1, ataxin-3, and the androgen receptor are cleaved in apoptotic extracts. Furthermore, each of these proteins is cleaved by one or more purified caspases, cysteine proteases involved in apoptotic death. The CAG length does not modulate susceptibility to cleavage of any of the full-length proteins. Our results suggest that by generation of truncated polyglutamine-containing proteins, caspase cleavage may represent a common step in the pathogenesis of each of these neurodegenerative diseases.

Of molecular interactions, mice and mechanisms: new insights into Huntington's disease

Huntington's disease is caused by expansion of a CAG trinucleotide beyond 35 repeats within the coding region of a novel gene. Recently, new insights into the relationship between CAG expansion in the HD gene and pathological mechanisms have emerged. These include a more precise understanding of the relationship between CAG repeat length and age of onset, progress in transgenic and excitotoxic animal models, identification of a novel huntington-interacting protein, and intriguing connections between huntington and the apoptotic machinery. We have combined many of these new findings into a model that suggests mechanisms and predicts outcomes by which the pathogenesis of Huntington's disease may be initiated. The development of appropriate in-vitro and animal models for Huntington's disease will allow the validity of this model to be tested.

Toward understanding the molecular pathology of Huntington's disease

Huntington's Disease (HD) is caused by expansion of a CAG trinucleotide beyond 35 repeats within the coding region of a novel gene. Recently, new insights into the relationship between CAG expansion in the HD gene and pathological mechanisms have emerged. Survival analysis of a large cohort of affected and at-risk individuals with CAG sizes between 39 and 50 repeats have yielded probability curves of developing HD symptoms and dying of HD by a certain age. Animals transgenic for the first exon of huntingtin with large CAG repeats lengths have been reported to have a complex neurological phenotype that bears interesting similarities and differences to HD. The repertoire of huntingtin-interacting proteins continues to expand with the identification of HIP1, a protein whose yeast homologues have known functions in regulating events associated with the cytoskeleton. The ability of huntingtin to interact with two of its four known protein partners appears to be influenced by CAG length. Caspase 3 (apopain), a key cysteine protease known to play a seminal role in neural apoptosis, has also been demonstrated to specifically cleave huntingtin in a CAG length-dependent manner. Many of these features are combined in a model suggesting mechanisms by which the pathogenesis of HD may be initiated. The development of appropriate in vitro and animal models for HD will allow the validity of these models to be tested.

Multiple elements in the c-fos protein-coding region facilitate mRNA deadenylation and decay by a mechanism coupled to translation

The c-fos proto-oncogene transcript is one of the most labile mammalian mRNAs known. Rapid degradation of c-fos mRNA is mediated by both the c-fos protein-coding region and an AU-rich element in the 3'-untranslated region. Here we present evidence that the c-fos coding region contains multiple destabilizing elements that can function independently to facilitate both deadenylation and decay of mRNA. The ability of these coding region destabilizing elements to direct deadenylation and decay requires the assembly of ribosomes at the 5' end of this domain and, most likely, translation of the message.

The destabilizing elements in the coding region of c-fos mRNA are recognized as RNA

The protein-coding region of the c-fos proto-oncogene transcript contains elements that direct the rapid deadenylation and decay of this mRNA in mammalian cells. The function of these coding region instability determinants requires movement of ribosomes across mRNAs containing them. Three types of mechanisms could account for this translational requirement. Two of these possibilities, (i) that rapid mRNA decay might be mediated by the nascent polypeptide chain and (ii) that it might result from an unusual codon usage, have experimental precedent. Here, we present evidence that the destabilizing elements in the c-fos coding region are not recognized in either of these two ways. Instead, the ability of the c-fos coding region to function as a potent mRNA destabilizer when translated in the +1 reading frame indicates that the signals for rapid deadenylation and decay reside in the sequence or structure of the RNA comprising this c-fos domain.

Functional significance of overlapping transcripts of crtEF, bchCA, and puf photosynthesis gene operons in Rhodobacter capsulatus

The Rhodobacter capsulatus crtEF, bchCA, and puf operons each encode products that function together to produce photosynthetic pigment-protein complexes. The crtEF operon encodes two enzymes of the carotenoid biosynthetic pathway, the bchCA operon encodes two enzymes of the bacteriochlorophyll biosynthetic pathway, and the puf operon contains six genes, four of which are known to code for proteins that bind pigment cofactors and position them in the intracytoplasmic membrane. These operons are adjacent on the chromosome and are transcribed in the same direction. Although each of these operons can be expressed from its own promoter, it is possible for transcription to begin at the crtEF promoter and continue through the bchCA and puf operons. We herein present the results of experiments which demonstrate that this overlapping transcriptional arrangement is important for establishment of normal levels of transcripts of the bchCA and puf operons and that read-through transcription from the bchCA operon contributes significantly to the expression of a pufB'::lac'Z gene fusion. Growth studies of crtF and bchC interposon mutants show that this read-through transcription is necessary for efficient transition from respiratory to photosynthetic growth modes.

Promoter mapping and nucleotide sequence of the bchC bacteriochlorophyll biosynthesis gene from Rhodobacter capsulatus

Because there are not yet direct assays for most of the proteins required for differentiation of Rhodobacter capsulatus cytoplasmic membrane into photosynthetically competent intracytoplasmic membrane, a molecular inquiry into the mechanism and regulation of this process is difficult. We have, therefore, chosen to isolate R. capsulatus photosynthesis genes by creating in-frame fusions to lac'Z vectors, and selecting for those that direct appropriately regulated levels of beta-galactosidase in R. capsulatus. One lacZ fusion isolate was used to identify an open reading frame (ORF) of unknown function and flanking sequences that promoted initiation of transcription. The chromosomal copy of this ORF was mutated by insertion of a kanamycin-resistant cartridge into the cloned fragment and substitution for the chromosomal copy by homologous recombination. The phenotype of the resultant mutant cells showed that the ORF encodes 2-desacetyl-2-hydroxyethyl bacteriochlorophyllide a dehydrogenase, an enzyme that catalyzes the penultimate step in bacteriochlorophyll a biosynthesis. The nucleotide sequence of this bchC gene and its 5' regulatory region were determined. The deduced amino acid sequence shows that the bchC gene encodes a 33-kDa protein that is less hydrophobic than integral membrane proteins of R. capsulatus, although there are hydrophobic segments that could in principle interact with a lipid membrane. Results of S1 nuclease protection and primer extension experiments show that a 5' mRNA end is positioned within the cloned segment, and that this 5' end maps to sequences with significant sequence similarity to the previously characterized puf operon promoter region.