Complementation of a yeast CYC3 deficiency identifies an X-linked mammalian activator of apocytochrome c

Unveiling the mechanism of efficient β-phenylethyl alcohol conversion in wild-type Saccharomyces cerevisiae WY319 through multi-omics analysis

β-Phenylethanol (2-PE), as an important flavor component in wine, is widely used in the fields of flavor chemistry and food health. 2-PE can be sustainably produced through Saccharomyces cerevisiae. Although significant progress has been made in obtaining high-yield strains, as well as improving the synthesis pathways of 2-PE, there still lies a gap between these two fields to unpin. In this study, the macroscopic metabolic characteristics of high-yield and low-yield 2-PE strains were systematically compared and analyzed. The results indicated that the production potential of the high-yield strain might be contributed to the enhancement of respiratory metabolism and the high tolerance to 2-PE. Furthermore, this hypothesis was confirmed through comparative genomics. Meanwhile, transcriptome analysis at key specific growth rates revealed that the collective upregulation of mitochondrial functional gene clusters plays a more prominent role in the production process of 2-PE. Finally, findings from untargeted metabolomics suggested that by enhancing respiratory metabolism and reducing the Crabtree effect, the accumulation of metabolites resisting high 2-PE stress was observed, such as intracellular amino acids and purines. Hence, this strategy provided a richer supply of precursors and cofactors, effectively promoting the synthesis of 2-PE. In short, this study provides a bridge for studying the metabolic mechanism of high-yield 2-PE strains with the subsequent targeted strengthening of relevant synthetic pathways. It also provides insights for the synthesis of nonalcoholic products in S. cerevisiae.

Linear Skin Defects with Multiple Congenital Anomalies (LSDMCA): An Unconventional Mitochondrial Disorder

Mitochondrial disorders, although heterogeneous, are traditionally described as conditions characterized by encephalomyopathy, hypotonia, and progressive postnatal organ failure. Here, we provide a systematic review of Linear Skin Defects with Multiple Congenital Anomalies (LSDMCA), a rare, unconventional mitochondrial disorder which presents as a developmental disease; its main clinical features include microphthalmia with different degrees of severity, linear skin lesions, and central nervous system malformations. The molecular basis of this disorder has been elusive for several years. Mutations were eventually identified in three X-linked genes, i.e., HCCS, COX7B, and NDUFB11, which are all endowed with defined roles in the mitochondrial respiratory chain. A peculiar feature of this condition is its inheritance pattern: X-linked dominant male-lethal. Only female or XX male individuals can be observed, implying that nullisomy for these genes is incompatible with normal embryonic development in mammals. All three genes undergo X-inactivation that, according to our hypothesis, may contribute to the extreme variable expressivity observed in this condition. We propose that mitochondrial dysfunction should be considered as an underlying cause in developmental disorders. Moreover, LSDMCA should be taken into consideration by clinicians when dealing with patients with microphthalmia with or without associated skin phenotypes.

Human Mitochondrial Pathologies of the Respiratory Chain and ATP Synthase: Contributions from Studies of Saccharomyces cerevisiae

The ease with which the unicellular yeast Saccharomyces cerevisiae can be manipulated genetically and biochemically has established this organism as a good model for the study of human mitochondrial diseases. The combined use of biochemical and molecular genetic tools has been instrumental in elucidating the functions of numerous yeast nuclear gene products with human homologs that affect a large number of metabolic and biological processes, including those housed in mitochondria. These include structural and catalytic subunits of enzymes and protein factors that impinge on the biogenesis of the respiratory chain. This article will review what is currently known about the genetics and clinical phenotypes of mitochondrial diseases of the respiratory chain and ATP synthase, with special emphasis on the contribution of information gained from pet mutants with mutations in nuclear genes that impair mitochondrial respiration. Our intent is to provide the yeast mitochondrial specialist with basic knowledge of human mitochondrial pathologies and the human specialist with information on how genes that directly and indirectly affect respiration were identified and characterized in yeast.

14-3-3ζ deficient mice in the BALB/c background display behavioural and anatomical defects associated with neurodevelopmental disorders

Sequencing and expression analyses implicate 14-3-3ζ as a genetic risk factor for neurodevelopmental disorders such as schizophrenia and autism. In support of this notion, we recently found that 14-3-3ζ^−/− mice in the Sv/129 background display schizophrenia-like defects. As epistatic interactions play a significant role in disease pathogenesis we generated a new congenic strain in the BALB/c background to determine the impact of genetic interactions on the 14-3-3ζ^−/− phenotype. In addition to replicating defects such as aberrant mossy fibre connectivity and impaired spatial memory, our analysis of 14-3-3ζ^−/− BALB/c mice identified enlarged lateral ventricles, reduced synaptic density and ectopically positioned pyramidal neurons in all subfields of the hippocampus. In contrast to our previous analyses, 14-3-3ζ^−/− BALB/c mice lacked locomotor hyperactivity that was underscored by normal levels of the dopamine transporter (DAT) and dopamine signalling. Taken together, our results demonstrate that dysfunction of 14-3-3ζ gives rise to many of the pathological hallmarks associated with the human condition. 14-3-3ζ-deficient BALB/c mice therefore provide a novel model to address the underlying biology of structural defects affecting the hippocampus and ventricle, and cognitive defects such as hippocampal-dependent learning and memory.

Mutations in NDUFB11, encoding a complex I component of the mitochondrial respiratory chain, cause microphthalmia with linear skin defects syndrome

Microphthalmia with linear skin defects (MLS) syndrome is an X-linked male-lethal disorder also known as MIDAS (microphthalmia, dermal aplasia, and sclerocornea). Additional clinical features include neurological and cardiac abnormalities. MLS syndrome is genetically heterogeneous given that heterozygous mutations in HCCS or COX7B have been identified in MLS-affected females. Both genes encode proteins involved in the structure and function of complexes III and IV, which form the terminal segment of the mitochondrial respiratory chain (MRC). However, not all individuals with MLS syndrome carry a mutation in either HCCS or COX7B. The majority of MLS-affected females have severe skewing of X chromosome inactivation, suggesting that mutations in HCCS, COX7B, and other as-yet-unidentified X-linked gene(s) cause selective loss of cells in which the mutated X chromosome is active. By applying whole-exome sequencing and filtering for X-chromosomal variants, we identified a de novo nonsense mutation in NDUFB11 (Xp11.23) in one female individual and a heterozygous 1-bp deletion in a second individual, her asymptomatic mother, and an affected aborted fetus of the subject's mother. NDUFB11 encodes one of 30 poorly characterized supernumerary subunits of NADH:ubiquinone oxidoreductase, known as complex I (cI), the first and largest enzyme of the MRC. By shRNA-mediated NDUFB11 knockdown in HeLa cells, we demonstrate that NDUFB11 is essential for cI assembly and activity as well as cell growth and survival. These results demonstrate that X-linked genetic defects leading to the complete inactivation of complex I, III, or IV underlie MLS syndrome. Our data reveal an unexpected role of cI dysfunction in a developmental phenotype, further underscoring the existence of a group of mitochondrial diseases associated with neurocutaneous manifestations.

Clinical spectrum of females with HCCS mutation: from no clinical signs to a neonatal lethal form of the microphthalmia with linear skin defects (MLS) syndrome

BACKGROUND

Segmental Xp22.2 monosomy or a heterozygous HCCS mutation is associated with the microphthalmia with linear skin defects (MLS) or MIDAS (microphthalmia, dermal aplasia, and sclerocornea) syndrome, an X-linked disorder with male lethality. HCCS encodes the holocytochrome c-type synthase involved in mitochondrial oxidative phosphorylation (OXPHOS) and programmed cell death.

METHODS

We characterized the X-chromosomal abnormality encompassing HCCS or an intragenic mutation in this gene in six new female patients with an MLS phenotype by cytogenetic analysis, fluorescence in situ hybridization, sequencing, and quantitative real-time PCR. The X chromosome inactivation (XCI) pattern was determined and clinical data of the patients were reviewed.

RESULTS

Two terminal Xp deletions of ≥ 11.2 Mb, two submicroscopic copy number losses, one of ~850 kb and one of ≥ 3 Mb, all covering HCCS, 1 nonsense, and one mosaic 2-bp deletion in HCCS are reported. All females had a completely (>98:2) or slightly skewed (82:18) XCI pattern. The most consistent clinical features were microphthalmia/anophthalmia and sclerocornea/corneal opacity in all patients and congenital linear skin defects in 4/6. Additional manifestations included various ocular anomalies, cardiac defects, brain imaging abnormalities, microcephaly, postnatal growth retardation, and facial dysmorphism. However, no obvious clinical sign was observed in three female carriers who were relatives of one patient.

CONCLUSION

Our findings showed a wide phenotypic spectrum ranging from asymptomatic females with an HCCS mutation to patients with a neonatal lethal MLS form. Somatic mosaicism and the different ability of embryonic cells to cope with an OXPHOS defect and/or enhanced cell death upon HCCS deficiency likely underlie the great variability in phenotypes.

Cytochrome c biogenesis System I: an intricate process catalyzed by a maturase supercomplex?

Cytochromes c are ubiquitous heme proteins that are found in most living organisms and are essential for various energy production pathways as well as other cellular processes. Their biosynthesis relies on a complex post-translational process, called cytochrome c biogenesis, responsible for the formation of stereo-specific thioether bonds between the vinyl groups of heme b (protoporphyrin IX-Fe) and the thiol groups of apocytochromes c heme-binding site (C1XXC2H) cysteine residues. In some organisms this process involves up to nine (CcmABCDEFGHI) membrane proteins working together to achieve heme ligation, designated the Cytochrome c maturation (Ccm)-System I. Here, we review recent findings related to the Ccm-System I found in bacteria, archaea and plant mitochondria, with an emphasis on protein interactions between the Ccm components and their substrates (apocytochrome c and heme). We discuss the possibility that the Ccm proteins may form a multi subunit supercomplex (dubbed "Ccm machine"), and based on the currently available data, we present an updated version of a mechanistic model for Ccm. This article is part of a Special Issue entitled: 18th European Bioenergetic Conference.

Structural and functional characterization of HP0377, a thioredoxin-fold protein from Helicobacter pylori

Maturation of cytochrome c is carried out in the bacterial periplasm, where specialized thiol-disulfide oxidoreductases provide the correct reduction of oxidized apocytochrome c before covalent haem attachment. HP0377 from Helicobacter pylori is a thioredoxin-fold protein that has been implicated as a component of system II for cytochrome c assembly and shows limited sequence similarity to Escherichia coli DsbC, a disulfide-bond isomerase. To better understand the role of HP0377, its crystal structures have been determined in both reduced and partially oxidized states, which are highly similar to each other. Sedimentation-equilibrium experiments indicate that HP0377 is monomeric in solution. HP0377 adopts a thioredoxin fold but shows distinctive variations as in other thioredoxin-like bacterial periplasmic proteins. The active site of HP0377 closely resembles that of E. coli DsbC. A reductase assay suggests that HP0377 may play a role as a reductase in the biogenesis of holocytochrome c553 (HP1227). Binding experiments indicate that it can form a covalent complex with HP0518, a putative L,D-transpeptidase with a catalytic cysteine residue, via a disulfide bond. Furthermore, physicochemical properties of HP0377 and its R86A variant have been determined. These results suggest that HP0377 may perform multiple functions as a reductase in H. pylori.

The impairment of HCCS leads to MLS syndrome by activating a non-canonical cell death pathway in the brain and eyes

Mitochondrial-dependent (intrinsic) programmed cell death (PCD) is an essential homoeostatic mechanism that selects bioenergetically proficient cells suitable for tissue/organ development. However, the link between mitochondrial dysfunction, intrinsic apoptosis and developmental anomalies has not been demonstrated to date. Now we provide the evidence that non-canonical mitochondrial-dependent apoptosis explains the phenotype of microphthalmia with linear skin lesions (MLS), an X-linked developmental disorder caused by mutations in the holo-cytochrome c-type synthase (HCCS) gene. By taking advantage of a medaka model that recapitulates the MLS phenotype we demonstrate that downregulation of hccs, an essential player of the mitochondrial respiratory chain (MRC), causes increased cell death via an apoptosome-independent caspase-9 activation in brain and eyes. We also show that the unconventional activation of caspase-9 occurs in the mitochondria and is triggered by MRC impairment and overproduction of reactive oxygen species (ROS). We thus propose that HCCS plays a key role in central nervous system (CNS) development by modulating a novel non-canonical start-up of cell death and provide the first experimental evidence for a mechanistic link between mitochondrial dysfunction, intrinsic apoptosis and developmental disorders.

Human mitochondrial holocytochrome c synthase's heme binding, maturation determinants, and complex formation with cytochrome c

Proper functioning of the mitochondrion requires the orchestrated assembly of respiratory complexes with their cofactors. Cytochrome c, an essential electron carrier in mitochondria and a critical component of the apoptotic pathway, contains a heme cofactor covalently attached to the protein at a conserved CXXCH motif. Although it has been known for more than two decades that heme attachment requires the mitochondrial protein holocytochrome c synthase (HCCS), the mechanism remained unknown. We purified membrane-bound human HCCS with endogenous heme and in complex with its cognate human apocytochrome c. Spectroscopic analyses of HCCS alone and complexes of HCCS with site-directed variants of cytochrome c revealed the fundamental steps of heme attachment and maturation. A conserved histidine in HCCS (His154) provided the key ligand to the heme iron. Formation of the HCCS:heme complex served as the platform for interaction with apocytochrome c. Heme was the central molecule mediating contact between HCCS and apocytochrome c. A conserved histidine in apocytochrome c (His19 of CXXCH) supplied the second axial ligand to heme in the trapped HCCS:heme:cytochrome c complex. We also examined the substrate specificity of human HCCS and converted a bacterial cytochrome c into a robust substrate for the HCCS. The results allow us to describe the molecular mechanisms underlying the HCCS reaction.

Mutations in COX7B cause microphthalmia with linear skin lesions, an unconventional mitochondrial disease

Microphthalmia with linear skin lesions (MLS) is an X-linked dominant male-lethal disorder associated with mutations in holocytochrome c-type synthase (HCCS), which encodes a crucial player of the mitochondrial respiratory chain (MRC). Unlike other mitochondrial diseases, MLS is characterized by a well-recognizable neurodevelopmental phenotype. Interestingly, not all clinically diagnosed MLS cases have mutations in HCCS, thus suggesting genetic heterogeneity for this disorder. Among the possible candidates, we analyzed the X-linked COX7B and found deleterious de novo mutations in two simplex cases and a nonsense mutation, which segregates with the disease, in a familial case. COX7B encodes a poorly characterized structural subunit of cytochrome c oxidase (COX), the MRC complex IV. We demonstrated that COX7B is indispensable for COX assembly, COX activity, and mitochondrial respiration. Downregulation of the COX7B ortholog (cox7B) in medaka (Oryzias latipes) resulted in microcephaly and microphthalmia that recapitulated the MLS phenotype and demonstrated an essential function of complex IV activity in vertebrate CNS development. Our results indicate an evolutionary conserved role of the MRC complexes III and IV for the proper development of the CNS in vertebrates and uncover a group of mitochondrial diseases hallmarked by a developmental phenotype.

Engineering a prokaryotic apocytochrome c as an efficient substrate for Saccharomyces cerevisiae cytochrome c heme lyase

Cytochromes c are heme proteins that require multiple maturation components, such as heme lyases, for cofactor incorporation. Saccharomyces cerevisiae has two heme lyases that are specific for apocytochromes c (CCHL) or c(1) (CC(1)HL). CCHL can covalently attach heme b groups to apocytochrome c substrates of eukaryotic but not prokaryotic origin. Besides their conserved Cys-Xxx-Xxx-Cys-His heme-binding motifs, the amino-terminal regions of apocytochrome c substrates appear to be important for CCHL function. In this study, we show for the first time that only two amino acid changes in the amino-terminal region of the non-CCHL substrate apocytochrome c(2) from Rhodobacter capsulatus are necessary and sufficient for efficient holocytochrome c formation by CCHL. This finding led us to propose a consensus sequence located at the amino-terminus of apocytochromes c, and critical for substrate recognition and heme ligation by CCHL.

Thiol redox requirements and substrate specificities of recombinant cytochrome c assembly systems II and III

The reconstitution of biosynthetic pathways from heterologous hosts can help define the minimal genetic requirements for pathway function and facilitate detailed mechanistic studies. Each of the three pathways for the assembly of cytochrome c in nature (called systems I, II, and III) has been shown to function recombinantly in Escherichia coli, covalently attaching heme to the cysteine residues of a CXXCH motif of a c-type cytochrome. However, recombinant systems I (CcmABCDEFGH) and II (CcsBA) function in the E. coli periplasm, while recombinant system III (CCHL) attaches heme to its cognate receptor in the cytoplasm of E. coli, which makes direct comparisons between the three systems difficult. Here we show that the human CCHL (with a secretion signal) attaches heme to the human cytochrome c (with a signal sequence) in the E. coli periplasm, which is bioenergetically (p-side) analogous to the mitochondrial intermembrane space. The human CCHL is specific for the human cytochrome c, whereas recombinant system II can attach heme to multiple non-cognate c-type cytochromes (possessing the CXXCH motif.) We also show that the recombinant periplasmic systems II and III use components of the natural E. coli periplasmic DsbC/DsbD thiol-reduction pathway. This article is part of a Special Issue entitled: Biogenesis/Assembly of Respiratory Enzyme Complexes.

Cytochrome c biogenesis in mitochondria--Systems III and V

In c-type cytochromes, heme becomes covalently attached to the polypeptide chain by a reaction between the vinyl groups of the heme and cysteine thiols from the protein. There are two such cytochromes in mitochondria: cytochrome c and cytochrome c(1). The heme attachment is a post-translational modification that is catalysed by different biogenesis proteins in different organisms. Three types of biogenesis system are found or predicted in mitochondria: System I (the cytochrome c maturation system); System III (termed holocytochrome c synthase (HCCS) or heme lyase); and System V. This review focuses primarily on cytochrome c maturation in mitochondria containing HCCS (System III). It describes what is known about the enzymology and substrate specificity of HCCS; the role of HCCS in human disease; import of HCCS into mitochondria; import of apocytochromes c and c(1) into mitochondria and the close relationships with HCCS-dependent heme attachment; and the role of the fungal cytochrome c biogenesis accessory protein Cyc2. System V is also discussed; this is the postulated mitochondrial cytochrome c biogenesis system of trypanosomes and related organisms. No cytochrome c biogenesis proteins have been identified in the genomes of these organisms whose c-type cytochromes also have a unique mode of heme attachment.

c-type cytochrome assembly in Saccharomyces cerevisiae: a key residue for apocytochrome c1/lyase interaction

The electron transport chains in the membranes of bacteria and organelles generate proton-motive force essential for ATP production. The c-type cytochromes, defined by the covalent attachment of heme to a CXXCH motif, are key electron carriers in these energy-transducing membranes. In mitochondria, cytochromes c and c(1) are assembled by the cytochrome c heme lyases (CCHL and CC(1)HL) and by Cyc2p, a putative redox protein. A cytochrome c(1) mutant with a CAPCH heme-binding site instead of the wild-type CAACH is strictly dependent upon Cyc2p for assembly. In this context, we found that overexpression of CC(1)HL, as well as mutations of the proline in the CAPCH site to H, L, S, or T residues, can bypass the absence of Cyc2p. The P mutation was postulated to shift the CXXCH motif to an oxidized form, which must be reduced in a Cyc2p-dependent reaction before heme ligation. However, measurement of the redox midpoint potential of apocytochrome c(1) indicates that neither the P nor the T residues impact the thermodynamic propensity of the CXXCH motif to occur in a disulfide vs. dithiol form. We show instead that the identity of the second intervening residue in the CXXCH motif is key in determining the CCHL-dependent vs. CC(1)HL-dependent assembly of holocytochrome c(1). We also provide evidence that Cyc2p is dedicated to the CCHL pathway and is not required for the CC(1)HL-dependent assembly of cytochrome c(1).

Compensatory growth of healthy cardiac cells in the presence of diseased cells restores tissue homeostasis during heart development

Energy generation by mitochondrial respiration is an absolute requirement for cardiac function. Here, we used a heart-specific conditional knockout approach to inactivate the X-linked gene encoding Holocytochrome c synthase (Hccs), an enzyme responsible for activation of respiratory cytochromes c and c1. Heterozygous knockout female mice were thus mosaic for Hccs function due to random X chromosome inactivation. In contrast to midgestational lethality of Hccs knockout males, heterozygous females appeared normal after birth. Analyses of heterozygous embryos revealed the expected 50:50 ratio of Hccs deficient to normal cardiac cells at midgestation; however, diseased tissue contributed progressively less over time and by birth represented only 10% of cardiac tissue volume. This change is accounted for by increased proliferation of remaining healthy cardiac cells resulting in a fully functional heart. These data reveal an impressive regenerative capacity of the fetal heart that can compensate for an effective loss of 50% of cardiac tissue.

Mutations of the mitochondrial holocytochrome c-type synthase in X-linked dominant microphthalmia with linear skin defects syndrome

The microphthalmia with linear skin defects syndrome (MLS, or MIDAS) is an X-linked dominant male-lethal disorder almost invariably associated with segmental monosomy of the Xp22 region. In two female patients, from two families, with MLS and a normal karyotype, we identified heterozygous de novo point mutations--a missense mutation (p.R217C) and a nonsense mutation (p.R197X)--in the HCCS gene. HCCS encodes the mitochondrial holocytochrome c-type synthase that functions as heme lyase by covalently adding the prosthetic heme group to both apocytochrome c and c(1). We investigated a third family, displaying phenotypic variability, in which the mother and two of her daughters carry an 8.6-kb submicroscopic deletion encompassing part of the HCCS gene. Functional analysis demonstrates that both mutant proteins (R217C and Delta 197-268) were unable to complement a Saccharomyces cerevisiae mutant deficient for the HCCS orthologue Cyc3p, in contrast to wild-type HCCS. Moreover, ectopically expressed HCCS wild-type and the R217C mutant protein are targeted to mitochondria in CHO-K1 cells, whereas the C-terminal-truncated Delta 197-268 mutant failed to be sorted to mitochondria. Cytochrome c, the final product of holocytochrome c-type synthase activity, is implicated in both oxidative phosphorylation (OXPHOS) and apoptosis. We hypothesize that the inability of HCCS-deficient cells to undergo cytochrome c-mediated apoptosis may push cell death toward necrosis that gives rise to severe deterioration of the affected tissues. In summary, we suggest that disturbance of both OXPHOS and the balance between apoptosis and necrosis, as well as the X-inactivation pattern, may contribute to the variable phenotype observed in patients with MLS.

Defects in the biosynthesis of mitochondrial heme c and heme a in yeast and mammals

Defects in heme biosynthesis have been associated with a large number of diseases, but mostly recognized in porphyrias, which are neurovisceral or cutaneous disorders caused by the accumulation of biosynthetic intermediates. However, defects in the maturation of heme groups that are part of the oxidative phosphorylation system are now also recognized as important causes of disease. The electron transport chain contains heme groups of the types a, b and c, all of which are directly involved in electron transfer reactions. In this article, we review the effect of mutations in enzymes involved in the maturation of heme a (the prosthetic group of cytochrome c oxidase) and heme c (the prosthetic group of cytochrome c) both in yeast and in humans. COX10 and COX15 are two genes, initially identified in Saccharomyces cerevisiae that have been found to cause infantile cytochrome c oxidase deficiency in humans. They participate in the farnesylation and hydroxylation of heme b, steps that are necessary for the formation of heme a, the prosthetic group required for cytochrome oxidase assembly and activity. Deletion of the cytochrome c heme lyase gene in a single allele has also been associated with a human disease, known as Microphthalmia with Linear Skin defects (MLS) syndrome. The cytochrome c heme lyase is necessary to covalently attach the heme group to the apocytochrome c polypeptide. The production of mouse models recapitulating these diseases is providing novel information on the pathogenesis of clinical syndromes.

Microphthalmia with linear skin defects syndrome (MLS): a male with a mosaic paracentric inversion of Xp

The microphthalmia with linear skin defects syndrome (MLS) is an X-linked dominant disorder with male lethality. In the majority of the patients reported, the MLS syndrome is caused by segmental monosomy of the Xp22.3 region. To date, five male patients with MLS and 46,XX karyotype ("XX males") have been described. Here we report on the first male case with MLS and an XY complement. The patient showed agenesis of the corpus callosum, histiocytoid cardiomyopathy, and lactic acidosis but no microphthalmia, and carried a mosaic subtle inversion of the short arm of the X chromosome in 15% of his peripheral blood lymphocytes, 46,Y,inv(X)(p22.13 approximately 22.2p22.32 approximately 22.33)[49]/46,XY[271]. By fluorescence IN SITU hybridization (FISH), we showed that YAC 225H10 spans the breakpoint in Xp22.3. End-sequencing and database analysis revealed a YAC insert of at least 416 kb containing the genes HCCS and AMELX, and exons 2-16 of ARHGAP6. Molecular cytogenetic data suggest that the Xp22.3 inversion breakpoint is located in intron 1 of ARHGAP6, the gene encoding the Rho GTPase activating protein 6. Future molecular studies in karyotypically normal female MLS patients to detect submicroscopic rearrangements including the ARHGAP6 gene as well as mutation screening of ARHGAP6 in patients with no obvious chromosomal rearrangements will clarify the role of this gene in MLS syndrome.

Overlapping specificities of the mitochondrial cytochrome c and c1 heme lyases

Heme attachment to the apoforms of fungal mitochondrial cytochrome c and c1 requires the activity of cytochrome c and c1 heme lyases (CCHL and CC1HL), which are enzymes with distinct substrate specificity. However, the presence of a single heme lyase in higher eukaryotes is suggestive of broader substrate specificity. Here, we demonstrate that yeast CCHL is active toward the non-cognate substrate apocytochrome c1, i.e. CCHL promotes low levels of apocytochrome c1 conversion to its holoform in the absence of CC1HL. Moreover, that the single human heme lyase also displays a broader cytochrome specificity is evident from its ability to substitute for both yeast CCHL and CC1HL. Multicopy and genetic suppressors of the absence of CC1HL were isolated and their analysis revealed that the activity of CCHL toward cytochrome c1 can be enhanced by: 1) reducing the abundance of the cognate substrate apocytochrome c, 2) increasing the accumulation of CCHL, 3) modifying the substrate-enzyme interaction through point mutations in CCHL or cytochrome c1, or 4) overexpressing Cyc2p, a protein known previously only as a mitochondrial biogenesis factor. Based on the functional interaction of Cyc2p with CCHL and the presence of a putative FAD-binding site in the protein, we hypothesize that Cyc2p controls the redox chemistry of the heme lyase reaction.