A 11.5 A single particle reconstruction of GroEL using EMAN.

Single-particle analysis has become an increasingly important method for structural determination of large macromolecular assemblies. GroEL is an 800 kDa molecular chaperone, which, along with its co-chaperonin GroES, promotes protein folding both in vitro and in the bacterial cell. EMAN is a single-particle analysis software package, which was first publicly distributed in 2000. We present a three-dimensional reconstruction of native naked GroEL to approximately 11.5 A performed entirely with EMAN. We demonstrate that the single-particle reconstruction, X-ray scattering data and X-ray crystal structure all agree well at this resolution. These results validate the specific methods of image restoration, reconstruction and evaluation techniques implemented in EMAN. It also demonstrates that the single-particle reconstruction technique and X-ray crystallography will yield consistent structure factors, even at low resolution, when image restoration is performed correctly. A detailed comparison of the single-particle and X-ray structures exhibits some small variations in the equatorial domain of the molecule, likely due to the absence of crystal packing forces in the single-particle reconstruction.

Structure of rat BCKD kinase: nucleotide-induced domain communication in a mitochondrial protein kinase.

Mitochondrial protein kinases (mPKs) are molecular switches that down-regulate the oxidation of branched-chain alpha-ketoacids and pyruvate. Elevated levels of these metabolites are implicated in disease states such as insulin-resistant Type II diabetes, branched-chain ketoaciduria, and primary lactic acidosis. We report a three-dimensional structure of a member of the mPK family, rat branched-chain alpha-ketoacid dehydrogenase kinase (BCK). BCK features a characteristic nucleotide-binding domain and a four-helix bundle domain. These two domains are reminiscent of modules found in protein histidine kinases (PHKs), which are involved in two-component signal transduction systems. Unlike PHKs, BCK dimerizes through direct interaction of two opposing nucleotide-binding domains. Nucleotide binding to BCK is uniquely mediated by both potassium and magnesium. Binding of ATP induces disorder-order transitions in a loop region at the nucleotide-binding site. These structural changes lead to the formation of a quadruple aromatic stack in the interface between the nucleotide-binding domain and the four-helix bundle domain, where they induce a movement of the top portion of two helices. Phosphotransfer induces further ordering of the loop region, effectively trapping the reaction product ADP, which explains product inhibition in mPKs. The BCK structure is a prototype for all mPKs and will provide a framework for structure-assisted inhibitor design for this family of kinases.

Natural osmolyte trimethylamine N-oxide corrects assembly defects of mutant branched-chain alpha-ketoacid decarboxylase in maple syrup urine disease.

Maple syrup urine disease is caused by deficiency in the mitochondrial branched-chain alpha-ketoacid dehydrogenase (BCKD) complex. The clinical phenotype includes often fatal ketoacidosis, neurological derangement, and mental retardation. The type IA mutations Y393N-alpha, Y368C-alpha, and F364C-alpha, which occur in the E1alpha subunit of the decarboxylase (E1) component of the BCKD complex, impede the conversion of an alphabeta heterodimeric intermediate to a native alpha(2)beta(2) heterotetramer in the E1 assembly pathway. In the present study, we show that a natural osmolyte trimethylamine N-oxide (TMAO) at the optimal 1 m concentration restores E1 activity, up to 50% of the wild type, in the mutant E1 carrying the above missense mutations. TMAO promotes the conversion of otherwise trapped mutant heterodimers to active heterotetramers. This slow step does not involve dissociation/reassociation of the mutant heterodimers, which are preformed in the presence of chaperonins GroEL/GroES and Mg-ATP. The TMAO-stimulated mutant E1 activity is remarkably stable upon removal of the osmolyte, when cofactor thiamine pyrophosphate and the transacylase component of the BCKD complex are present. The above in vitro results offer the use of chemical chaperones such as TMAO as an approach to mitigate assembly defects caused by maple syrup urine disease mutations.

Biochemical basis of type IB (E1beta ) mutations in maple syrup urine disease. A prevalent allele in patients from the Druze kindred in Israel.

Maple syrup urine disease (MSUD) is a metabolic disorder associated with often-fatal ketoacidosis, neurological derangement, and mental retardation. In this study, we identify and characterize two novel type IB MSUD mutations in Israeli patients, which affect the E1beta subunit in the decarboxylase (E1) component of the branched-chain alpha-ketoacid dehydrogenase complex. The recombinant mutant E1 carrying the prevalent S289L-beta (TCG --> TTG) mutation in the Druze kindred exists as a stable inactive alphabeta heterodimer. Based on the human E1 structure, the S289L-beta mutation disrupts the interactions between Ser-289-beta and Glu-290-beta', and between Arg-309-beta and Glu-290-beta', which are essential for native alpha(2)beta(2) heterotetrameric assembly. The R133P-beta (CGG --> CCG) mutation, on the other hand, is inefficiently expressed in Escherichia coli as heterotetramers in a temperature-dependent manner. The R133P-beta mutant E1 exhibits significant residual activity but is markedly less stable than the wild-type, as measured by thermal inactivation and free energy change of denaturation. The R133P-beta substitution abrogates the coordination of Arg-133-beta to Ala-95-beta, Glu-96-beta, and Ile-97-beta, which is important for strand-strand interactions and K(+) ion binding in the beta subunit. These findings provide new insights into folding and assembly of human E1 and will facilitate DNA-based diagnosis for MSUD in the Israeli population.

Roles of active site and novel K+ ion-binding site residues in human mitochondrial branched-chain alpha-ketoacid decarboxylase/dehydrogenase.

The human mitochondrial branched-chain alpha-ketoacid decarboxylase/dehydrogenase (BCKD) is a heterotetrameric (alpha(2)beta(2)) thiamine diphosphate (TDP)-dependent enzyme. The recently solved human BCKD structure at 2.7 A showed that the two TDP-binding pockets are located at the interfaces between alpha and beta' subunits and between alpha' and beta subunits. In the present study, we show that the E76A-beta' mutation results in complete inactivation of BCKD. The result supports the catalytic role of the invariant Glu-76-beta' residue in increasing basicity of the N-4' amino group during the proton abstraction from the C-2 atom on the thiazolium ring. A substitution of His-146-beta' with Ala also renders the enzyme completely inactive. The data are consistent with binding of the alpha-ketoacid substrate by this residue based on the Pseudomonas BCKD structure. Alterations in Asn-222-alpha, Tyr-224-alpha, or Glu-193-alpha, which coordinates to the Mg(2+) ion, result in an inactive enzyme (E193A-alpha) or a mutant BCKD with markedly higher K(m) for TDP and a reduced level of the bound cofactor (Y224A-alpha and N222S-alpha). Arg-114-alpha, Arg-220-alpha, and His-291-alpha interact with TDP by directly binding to phosphate oxygens of the cofactor. We show that natural mutations of these residues in maple syrup urine disease (MSUD) patients (R114W-alpha and R220W-alpha) or site-directed mutagenesis (H291A-alpha) also result in an inactive or partially active enzyme, respectively. Another MSUD mutation (T166M-alpha), which affects one of the residues that coordinate to the K(+) ion on the alpha subunit, also causes inactivation of the enzyme and an attenuated ability to bind TDP. In addition, fluorescence measurements establish that Trp-136-beta in human BCKD is the residue quenched by TDP binding. Thus, our results define the functional roles of key amino acid residues in human BCKD and provide a structural basis for MSUD.

Rare etiology of autosomal recessive disease in a child with noncarrier parents.

A child with maple syrup urine disease type 2 (MSUD2) was found to be homozygous for a 10-bp MSUD2-gene deletion on chromosome 1. Both purported parents were tested, and neither carries the gene deletion. Polymorphic simple-sequence repeat analyses at 15 loci on chromosome 1 and at 16 loci on other chromosomes confirmed parentage and revealed that a de novo mutation prior to maternal meiosis I, followed by nondisjunction in maternal meiosis II, resulted in an oocyte with two copies of the de novo mutant allele. Fertilization by a sperm that did not carry a paternal chromosome 1 or subsequent mitotic loss of the paternal chromosome 1 resulted in the propositus inheriting two mutant MSUD2 alleles on two maternal number 1 chromosomes.

Tetrameric assembly and conservation in the ATP-binding domain of rat branched-chain alpha-ketoacid dehydrogenase kinase.

We showed previously that the rat branched-chain alpha-ketoacid dehydrogenase (BCKD) kinase is capable of autophosphorylation. However, despite its sequence similarity to bacterial histidine protein kinases, BCKD kinase does not function as a histidine protein kinase. In the present study, we report that the rat BCKD kinase exists as a homotetramer of M(r) = 185,000, based on results of gel filtration and dynamic light scattering. This is in contrast to the related mammalian pyruvate dehydrogenase kinase isozymes that occur as homodimers. The tetrameric assembly of BCKD kinase was confirmed by the presence of four 5'-adenylyl-imidodiphosphate-binding sites (K(D) = 4.1 x 10(-6)m) per molecule of the kinase. Incubation of the BCKD kinase with increasing concentrations of urea resulted in dissociation of the tetramer to dimers and eventually to monomers as separated on a sucrose density gradient. Both tetramers and dimers, but not the monomer, maintained the conformation capable of binding ATP and undergoing autophosphorylation. BCKD kinase depends on a fully lipoylated transacylase for maximal activity, but the interaction between the kinase and the transacylase is impeded in the presence of high salt concentrations. Alterations of conserved residues in the ATP-binding domain led to a marked reduction or complete loss in the catalytic efficiency of the BCKD kinase. The results indicate that BCKD kinase, similar to pyruvate dehydrogenase kinase isozymes, belongs to the superfamily of ATPase/kinase.

Crystal structure of human branched-chain alpha-ketoacid dehydrogenase and the molecular basis of multienzyme complex deficiency in maple syrup urine disease.

BACKGROUND: Mutations in components of the extraordinarily large alpha-ketoacid dehydrogenase multienzyme complexes can lead to serious and often fatal disorders in humans, including maple syrup urine disease (MSUD). In order to obtain insight into the effect of mutations observed in MSUD patients, we determined the crystal structure of branched-chain alpha-ketoacid dehydrogenase (E1), the 170 kDa alpha(2)beta(2) heterotetrameric E1b component of the branched-chain alpha-ketoacid dehydrogenase multienzyme complex. RESULTS: The 2.7 A resolution crystal structure of human E1b revealed essentially the full alpha and beta polypeptide chains of the tightly packed heterotetramer. The position of two important potassium (K(+)) ions was determined. One of these ions assists a loop that is close to the cofactor to adopt the proper conformation. The second is located in the beta subunit near the interface with the small C-terminal domain of the alpha subunit. The known MSUD mutations affect the functioning of E1b by interfering with the cofactor and K(+) sites, the packing of hydrophobic cores, and the precise arrangement of residues at or near several subunit interfaces. The Tyr-->Asn mutation at position 393-alpha occurs very frequently in the US population of Mennonites and is located in a unique extension of the human E1b alpha subunit, contacting the beta' subunit. CONCLUSIONS: Essentially all MSUD mutations in human E1b can be explained on the basis of the structure, with the severity of the mutations for the stability and function of the protein correlating well with the severity of the disease for the patients. The suggestion is made that small molecules with high affinity for human E1b might alleviate effects of some of the milder forms of MSUD.

Interactions of GroEL/GroES with a heterodimeric intermediate during alpha 2beta 2 assembly of mitochondrial branched-chain alpha-ketoacid dehydrogenase. cis capping of the native-like 86-kDa intermediate by GroES.

We showed previously that the interaction of an alphabeta heterodimeric intermediate with GroEL/GroES is essential for efficient alpha(2)beta(2) assembly of human mitochondrial branched-chain alpha-ketoacid dehydrogenase. In the present study, we further characterized the mode of interaction between the chaperonins and the native-like alphabeta heterodimer. The alphabeta heterodimer, as an intact entity, was found to bind to GroEL at a 1:1 stoichiometry with a K(D) of 1.1 x 10(-)(7) m. The 1:1 molar ratio of the GroEL-alphabeta complex was confirmed by the ability of the complex to bind a stoichiometric amount of denatured lysozyme in the trans cavity. Surprisingly, in the presence of Mg-ADP, GroES was able to cap the GroEL-alphabeta complex in cis, despite the size of 86 kDa of the heterodimer (with a His(6) tag and a linker). Incubation of the GroEL-alphabeta complex with Mg-ATP, but not AMP-PNP, resulted in the release of alpha monomers. In the presence of Mg-ATP, the beta subunit was also released but was unable to assemble with the alpha subunit, and rebound to GroEL. The apparent differential subunit release from GroEL is explained, in part, by the significantly higher binding affinity of the beta subunit (K(D) < 4.15 x 10(-9)m) than the alpha (K(D) = 1.6 x 10(-8)m) for GroEL. Incubation of the GroEL-alphabeta complex with Mg-ATP and GroES resulted in dissociation and discharge of both the alpha and beta subunits from GroEL. The beta subunit upon binding to GroEL underwent further folding in the cis cavity sequestered by GroES. This step rendered the beta subunit competent for reassociation with the soluble alpha subunit to produce a new heterodimer. We propose that this mechanism is responsible for the iterative annealing of the kinetically trapped heterodimeric intermediate, leading to an efficient alpha(2)beta(2) assembly of human branched-chain alpha-ketoacid dehydrogenase.

GroEL/GroES promote dissociation/reassociation cycles of a heterodimeric intermediate during alpha(2)beta(2) protein assembly. Iterative annealing at the quaternary structure level.

Whereas the mechanism of GroEL/GroES-mediated protein folding has been extensively studied, the role of these chaperonins in oligomeric protein assembly remains poorly understood. In the present study, we investigated the interaction of the chaperonins with an alphabeta heterodimeric intermediate during the alpha(2)beta(2) assembly of human mitochondrial branched-chain alpha-ketoacid dehydrogenase/decarboxylase (BCKD). Incubation of the recombinant His(6)-tagged BCKD in 400 mM KSCN for 45 min at 23 degrees C caused a complete dissociation of the alpha(2)beta(2) heterotetramers into inactive alphabeta heterodimers. Dilution of the denaturant resulted in a rapid recovery of BCKD independent of the chaperonins GroEL/GroES. Prolonged incubation of BCKD in 400 mM KSCN resulted in the generation of nonproductive or "bad" heterodimers, which were unable to undergo spontaneous reactivation but capable of binding to GroEL to form a stable GroEL-alphabeta complex. Incubation of this complex with GroES and Mg-ATP led to the slow reactivation of BCKD with a second-order rate constant k = 480 M(-1) s(-1). Mixing experiments with radiolabeled and unlabeled protein substrates provided direct evidence that GroEL/GroES promote dissociation and subunit exchange between bad heterodimers. This was accompanied by the transformation of bad heterodimers to their "good" or productive counterparts. The good heterodimers were capable of spontaneous dimerization to initially form an inactive heterotetrameric species, followed by conversion to active heterotetramers. However, a large fraction of bad heterodimers were regenerated and rebound to GroEL. The cycle was perpetuated until the reconstitution of active BCKD was complete. Our data support the thesis that chaperonins GroEL/GroES mediate iterative annealing of nonproductive assembly intermediates at the quaternary structure level. This step is essential for an efficient subsequent higher order oligomerization.

Down-regulation of rat mitochondrial branched-chain 2-oxoacid dehydrogenase kinase gene expression by glucocorticoids.

The mammalian mitochondrial branched-chain 2-oxoacid dehydrogenase (BCOD) complex is regulated by a reversible phosphorylation (inactivation)/dephosphorylation (activation) cycle. In the present study, the effects of glucocorticoids on the level of BCOD kinase mRNA were investigated in rat hepatoma cell lines (H4IIE and FTO-2B), as well as in the rat. In H4IIE cells, dexamethasone was found to significantly reduce steady-state concentrations of BCOD kinase mRNA after a 48 h culture, and this was correlated with a 2-fold increase in the dephosphorylated form of the BCOD complex. The half-life of the kinase mRNA in H4IIE cells was not affected by dexamethasone treatment. Therefore, the decrease in the steady-state kinase mRNA level resulting from dexamethasone treatment was not caused by changes in mRNA stability, which raised the possibility of regulation at the level of gene transcription. To identify the negative glucocorticoid-responsive element in the kinase promoter, nested deletion constucts in the 3.0 kb promoter region were examined in H4IIE cells cultured in the presence or absence of dexamethasone. No significant differences in promoter activity were observed on either transient or stable transfection. The data showed that the glucocorticoid-responsive element was located outside the 3. 0 kb promoter region. At the physiological level, hepatic BCOD kinase mRNA levels were reduced in rats injected intraperitoneally with dexamethasone. This effect was liver-specific, and was not detected in other tissues. These results suggest that the down-regulation of kinase gene expression by glucocorticoids is mediated through a liver-specific or -enriched transcription factor(s).

GroEL/GroES-dependent reconstitution of alpha2 beta2 tetramers of humanmitochondrial branched chain alpha-ketoacid decarboxylase. Obligatory interaction of chaperonins with an alpha beta dimeric intermediate.

The decarboxylase component (E1) of the human mitochondrial branched chain alpha-ketoacid dehydrogenase multienzyme complex (approximately 4-5 x 10(3) kDa) is a thiamine pyrophosphate-dependent enzyme, comprising two 45.5-kDa alpha subunits and two 37.8-kDa beta subunits. In the present study, His6-tagged E1 alpha2 beta2 tetramers (171 kDa) denatured in 8 M urea were competently reconstituted in vitro at 23 degrees C with an absolute requirement for chaperonins GroEL/GroES and Mg-ATP. Unexpectedly, the kinetics for the recovery of E1 activity was very slow with a rate constant of 290 M-1 s-1. Renaturation of E1 with a similarly slow kinetics was also achieved using individual GroEL-alpha and GroEL-beta complexes as combined substrates. However, the beta subunit was markedly more prone to misfolding than the alpha in the absence of GroEL. The alpha subunit was released as soluble monomers from the GroEL-alpha complex alone in the presence of GroES and Mg-ATP. In contrast, the beta subunit discharged from the GroEL-beta complex readily rebound to GroEL when the alpha subunit was absent. Analysis of the assembly state showed that the His6-alpha and beta subunits released from corresponding GroEL-polypeptide complexes assembled into a highly structured but inactive 85.5-kDa alpha beta dimeric intermediate, which subsequently dimerized to produce the active alpha2 beta2 tetrameter. The purified alpha beta dimer isolated from Escherichia coli lysates was capable of binding to GroEL to produce a stable GroEL-alpha beta ternary complex. Incubation of this novel ternary complex with GroES and Mg-ATP resulted in recovery of E1 activity, which also followed slow kinetics with a rate constant of 138 M-1 s-1. Dimers were regenerated from the GroEL-alpha beta complex, but they needed to interact with GroEL/GroES again, thereby perpetuating the cycle until the conversion from dimers to tetramers was complete. Our study describes an obligatory role of chaperonins in priming the dimeric intermediate for subsequent tetrameric assembly, which is a slow step in the reconstitution of E1 alpha2 beta2 tetramers.

Mechanisms for GroEL/GroES-mediated folding of a large 86-kDa fusion polypeptide in vitro.

Our understanding of mechanisms for GroEL/GroES-assisted protein folding to date has been derived mostly from studies with small proteins. Little is known concerning the interaction of these chaperonins with large multidomain polypeptides during folding. In the present study, we investigated chaperonin-dependent folding of a large 86-kDa fusion polypeptide, in which the mature maltose-binding protein (MBP) sequence was linked to the N terminus of the alpha subunit of the decarboxylase (E1) component of the human mitochondrial branched-chain alpha-ketoacid dehydrogenase complex. The fusion polypeptide, MBP-alpha, when co-expressed with the beta subunit of E1, produced a chimeric protein MBP-E1 with an (MBP-alpha)2beta2 structure, similar to the alpha2 beta2 structure in native E1. Reactivation of MBP-E1 denatured in 8 M urea was absolutely dependent on GroEL/GroES and Mg2+-ATP, and exhibited strikingly slow kinetics with a rate constant of 376 M-1 s-1, analogous to denatured untagged E1. Chaperonin-mediated refolding of the MBP-alpha fusion polypeptide showed that the folding of the MBP moiety was about 7-fold faster than that of the alpha moiety on the same chain with rate constants of 1.9 x 10(-3) s-1 and 2.95 x 10(-4) s-1, respectively. This explained the occurrence of an MBP-alpha. GroEL binary complex that was isolated with amylose resin from the refolding mixture and transformed Escherichia coli lysates. The data support the thesis that distinct functional sequences in a large polypeptide exhibit different folding characteristics on the same GroEL scaffold. Moreover, we show that when the alpha.GroEL complex (molar ratio 1:1) was incubated with GroES, the latter was capable of capping either the very ring that harbored the 48-kDa (His)6-alpha polypeptide (in cis) or the opposite unoccupied cavity (in trans). In contrast, the MBP-alpha.GroEL (1:1) complex was capped by GroES exclusively in the trans configuration. These findings suggest that the productive folding of a large multidomain polypeptide can only occur in the GroEL cavity that is not sequestered by GroES.

Mechanism for basal expression of rat mitochondrial branched-chain-2-oxo-acid dehydrogenase kinase [corrected].

The rat branched-chain-2-oxo-acid dehydrogenase (BCOD) kinase mRNA is transcribed from a TATA-less promoter that has GC-rich sequences and two putative Sp1 binding sites near the transcription start site. We demonstrated previously that the 5' region of the kinase gene, base pairs -128 to +264, contained promoter activity when assayed using luciferase as a reporter (Huang and Chuang (1996) Biochem. J. 313, 603-609). To define DNA elements required for efficient expression of the kinase gene, nested deletion constructs of the above promoter region fused with a luciferase reporter gene were transfected into cultured H4IIE (hepatoma) and NRK-52E (kidney) cells. The results showed that the region between nucleotides -58 and +21 was indispensable for the kinase basal promoter activity. Methylation-interference and mutagenesis-promoter assays identified nucleotides -50 to -40 (ACAACTCCCA) as cis-acting DNA sequences that are required for nuclear protein binding and efficient promoter activity. Gel-supershift analysis with anti-Sp1 antibody suggested that the nuclear protein capable of binding to the -58 oligonucleotide (bp -58 to -34) was immunologically related to the Sp1 protein. The -58 oligonucleotide formed a DNA-protein complex with recombinant Sp1 protein with an affinity approximately ten-fold lower than that of the consensus Sp1 oligonucleotide. Co-transfection of the Sp1 expression plasmid and the -58 promoter construct into Drosophila Schneider cells revealed that Sp1 contributed to the kinase basal promoter activity by binding to the non-consensus site in the -58 region. Deletion of two consensus Sp1 binding sites (bases -150 to -140 and bases +29 to +38) in the kinase gene did not affect the basal promoter activity. Therefore binding of Sp1 or Sp1-like proteins to the above single non-consensus Sp1 sequence in the -58 region plays a major role of transactivating basal expression of the BCOD kinase.

Impaired assembly of E1 decarboxylase of the branched-chain alpha-ketoacid dehydrogenase complex in type IA maple syrup urine disease.

The E1 decarboxylase component of the human branched-chain ketoacid dehydrogenase complex comprises two E1alpha (45.5 kDa) and two E1beta (37.5 kDa) subunits forming an alpha2 beta2 tetramer. In patients with type IA maple syrup urine disease, the E1alpha subunit is affected, resulting in the loss of E1 and branched-chain ketoacid dehydrogenase catalytic activities. To study the effect of human E1alpha missense mutations on E1 subunit assembly, we have developed a pulse-chase labeling protocol based on efficient expression and assembly of human (His)6-E1alpha and untagged E1beta subunits in Escherichia coli in the presence of overexpressed chaperonins GroEL and GroES. Assembly of the two 35S-labeled E1 subunits was indicated by their co-extraction with Ni2+-nitrilotriacetic acid resin. The nine E1alpha maple syrup urine disease mutants studied showed aberrant kinetics of assembly with normal E1beta in the 2-h chase compared with the wild type and can be classified into four categories of normal (N222S-alpha and R220W-alpha), moderately slow (G245R-alpha), slow (G204S-alpha, A240P-alpha, F364C-alpha, Y368C-alpha, and Y393N-alpha), and no (T265R-alpha) assembly. Prolonged induction in E. coli grown in the YTGK medium or lowering of induction temperature from 37 to 28 degreesC (in the case of T265R-alpha), however, resulted in the production of mutant E1 proteins. Separation of purified E1 proteins by sucrose density gradient centrifugation showed that the wild-type E1 existed entirely as alpha2 beta2 tetramers. In contrast, a subset of E1alpha missense mutations caused the occurrence of exclusive alphabeta dimers (Y393N-alpha and F364C-alpha) or of both alpha2beta2 tetramers and lower molecular weight species (Y368C-alpha and T265R-alpha). Thermal denaturation at 50 degreesC indicated that mutant E1 proteins aggregated more rapidly than wild type (rate constant, 0.19 min-1), with the T265R-alpha mutant E1 most severely affected (rate constant, 4.45 min-1). The results establish that the human E1alpha mutations in the putative thiamine pyrophosphate-binding pocket that are studied, with the exception of G204S-alpha, have no effect on E1 subunit assembly. The T265R-alpha mutation adversely impacts both E1alpha folding and subunit interactions. The mutations involving the C-terminal aromatic residues impede both the kinetics of subunit assembly and the formation of the native alpha2 beta2 structure.

Maple syrup urine disease: it has come a long way.

Maple syrup urine disease (MSUD) was first described in 1954 by Menkes et al. as a progressive neurologic degenerative disorder. In 1960, Dancis et al. established that the metabolic block in MSUD is at the decarboxylation of branched-chain alpha-ketoacids derived from leucine, isoleucine, and valine. The multienzyme complex affected in MSUD, the mitrochondrial branched-chain alpha-ketoacid (BCKD) dehydrogenase complex was purified in 1978 to homogeneity in Reed's laboratory. This led to the later cloning of cDNAs and genes for subunits of the human BCKD complex. Genetic heterogeneity in MSUD is now explained by the various mutations that occur in the E1 alpha, E1 beta, E2, and E3 loci of the BCKD complex. Recently, we found that bacterial chaperonins GroEL and GroES promote folding and assembly of E1 decarboxylase component of the BCKD complex in Escherichia coli. Pulse-chase labeling in this system showed that a subset of E1 alpha mutations, notably the homozygous Y393N-alpha in Mennonite MSUD patients, impedes the assembly of the mutant E1 alpha subunit with normal E1 beta. The assembly defect is associated with a rapid degradation of the normal E1 beta subunit in MSUD cells. Retrovirus-mediated transduction of lymphoblasts from a Mennonite MSUD patient with a normal E1 alpha cDNA resulted in a complete restoration of BCKD activity. This was accompanied by a stabilization of the normal E1 beta subunit through assembly with recombinant E1 alpha. The results demonstrated the feasibility of stable correction of E1 alpha-deficient (type IA) MSUD and provided a basis for the development of gene therapy.