Structure of the Bateman2 domain of yeast Snf4: dimeric association and relevance for AMP binding

Improved Lipogenesis in Mortierella alpina by Abolishing the Snf4-Mediated Energy-Saving Mode under Low Glucose

Sensing nutrient levels and coordinating metabolism are requisites for all living organisms. In eukaryotes, heterotrimeric adenosine monophosphate-activated protein kinase/sucrose nonfermenting 1 (SNF1) is an energy monitor that primarily functions by regulating cell metabolism with its γ-subunit being responsible for energy sensing. Because of its strong lipogenesis capacity and dependence on nutrient availability, Mortierella alpina is an ideal model to investigate the SNF1 role. Knockdown of the M. alpina SNF1-γ-subunit (MaSnf4) abolished the energy preservation mode. In a low glucose medium (15 g/L), the fatty acid content in the MaSnf4-knockdown strain was similar to that in a high glucose medium (50 g/L), comprising 16 ± 1.17% of the dry cell weight after 96 h of culture (1.59 g/L), together with 1.41 ± 0.13 and 4.15 ± 0.19 fold increased acetyl-CoA carboxylase 1 and ATP-citrate lyase enzymatic activities, respectively. Metabolite analysis confirmed that knocking down MaSnf4 enhanced amino acid recycling and repressed the tricarboxylic acid cycle. In this case, more carbon skeleton acetyl-CoA and reductive nicotinamide adenine dinucleotide phosphate were rerouted into the fatty acid synthesis pathway. These findings provide new insight into the correlation between energy preservation and MaSnf4-regulated lipogenesis, which may enhance further development of cost-effective strategies to enhance lipid productivity in M. alpina.

Coding the α-subunit of SNF1 kinase, Snf1 is required for the conidiogenesis and pathogenicity of the Alternaria alternata tangerine pathotype

To well cope with various external carbon sources, fungi have evolved an adaptive mechanism to overcome the adversity of carbon source deficiency. The sucrose non-fermenting (SNF1) protein kinase mainly mediates the utilization of non-fermentable carbon sources. In this study, we determined the function of Snf1, coding the α-subunit of SNF1 kinase, in the phytopathogenic fungus Alternaria alternata via analyzing the Snf1 deletion mutants (ΔAasnf1). Aasnf1 is required for growth, development of aerial mycelium, and conidiation. Results of pathogenicity test showed that ΔAasnf1 induced smaller lesions on detached citrus leaves. Moreover, in the carbon utilization assay, ΔAasnf1 showed growth inhibition on the minimal medium supplemented with polygalacturonic acid, sucrose or alcohol as the only carbon source. Compared to the wild type, ΔAasnf1 also exhibited stronger resistance to cell wall stressors of sodium dodecyl sulfate and congo red. In conclusion, Aasnf1 played important roles in the carbon utilization, vegetative growth, conidiation, cell wall functions and pathogenicity of A. alternata. This study is the first report on the functions of Aasnf1 and our results suggest that Snf1 is critical for the conidiogenesis and pathogenesis of the A. alternata tangerine pathotype.

Structure and Regulation of AMPK

AMP-activated protein kinase is a family of heterotrimeric serine/threonine protein kinases that come in twelve different flavors. They serve an essential function in all eukaryotes of conserving cellular energy levels. AMPK complexes are regulated by changes in cellular AMP:ATP or ADP:ATP ratios and by a number of neutraceuticals and some of the widely-used diabetes medications such as metformin and thiazolinonediones. Moreover, biochemical activities of AMPK are tightly regulated by phosphorylation or dephosphorylation by upstream kinases and phosphatases respectively. Efforts are underway in many pharmaceutical companies to discover direct AMPK activators for the treatment of cardiovascular and metabolic diseases such as diabetes, non-alcoholic steatohepatitis (NASH) and diabetic nephropathy. Many advances have been made in the AMPK structural biology arena over the last few years that are beginning to provide detailed molecular insights into the overall topology of these fascinating enzymes and how binding of small molecules elicit subtle conformational changes leading to their activation and protection from dephosphorylation. In the brief review below on AMPK structure and function, we have focused on the recent crystallographic results especially on specific molecular interactions of direct synthetic AMPK activators which lead to biased activation of a sub-family of AMPK isoforms.

Mechanisms of regulation of SNF1/AMPK/SnRK1 protein kinases

The SNF1 (sucrose non-fermenting 1)-related protein kinases 1 (SnRKs1) are the plant orthologs of the budding yeast SNF1 and mammalian AMPK (AMP-activated protein kinase). These evolutionarily conserved kinases are metabolic sensors that undergo activation in response to declining energy levels. Upon activation, SNF1/AMPK/SnRK1 kinases trigger a vast transcriptional and metabolic reprograming that restores energy homeostasis and promotes tolerance to adverse conditions, partly through an induction of catabolic processes and a general repression of anabolism. These kinases typically function as a heterotrimeric complex composed of two regulatory subunits, β and γ, and an α-catalytic subunit, which requires phosphorylation of a conserved activation loop residue for activity. Additionally, SNF1/AMPK/SnRK1 kinases are controlled by multiple mechanisms that have an impact on kinase activity, stability, and/or subcellular localization. Here we will review current knowledge on the regulation of SNF1/AMPK/SnRK1 by upstream components, post-translational modifications, various metabolites, hormones, and others, in an attempt to highlight both the commonalities of these essential eukaryotic kinases and the divergences that have evolved to cope with the particularities of each one of these systems.

Change in single cystathionine β-synthase domain-containing protein from a bent to flat conformation upon adenosine monophosphate binding

Cystathionine β-synthase (CBS) domains are small intracellular modules that can act as binding domains for adenosine derivatives, and they may regulate the activity of associated enzymes or other functional domains. Among these, the single CBS domain-containing proteins, CBSXs, from Arabidopsis thaliana, have recently been identified as redox regulators of the thioredoxin system. Here, the crystal structure of CBSX2 in complex with adenosine monophosphate (AMP) is reported at 2.2Å resolution. The structure of dimeric CBSX2 with bound-AMP is shown to be approximately flat, which is in stark contrast to the bent form of apo-CBSXs. This conformational change in quaternary structure is triggered by a local structural change of the unique α5 helix, and by moving each loop P into an open conformation to accommodate incoming ligands. Furthermore, subtle rearrangement of the dimer interface triggers movement of all subunits, and consequently, the bent structure of the CBSX2 dimer becomes a flat structure. This reshaping of the structure upon complex formation with adenosine-containing ligand provides evidence that ligand-induced conformational reorganization of antiparallel CBS domains is an important regulatory mechanism.

Purification, crystallization and preliminary crystallographic analysis of the CBS-domain protein MJ1004 from Methanocaldococcus jannaschii

The purification and preliminary crystallographic analysis of the archaeal CBS-domain protein MJ1004 from Methanocaldococcus jannaschii are described. The native protein was overexpressed, purified and crystallized in the monoclinic space group P2(1), with unit-cell parameters a=54.4, b=53.8, c=82.6 Å, β=106.1°. The crystals diffracted X-rays to 2.7 Å resolution using synchrotron radiation. Matthews-volume calculations suggested the presence of two molecules in the asymmetric unit that are likely to correspond to a dimeric species, which is also observed in solution.

Unique gating properties of C. elegans ClC anion channel splice variants are determined by altered CBS domain conformation and the R-helix linker

All eukaryotic and some prokaryotic ClC anion transport proteins have extensive cytoplasmic C-termini containing two cystathionine-β-synthase (CBS) domains. CBS domain secondary structure is highly conserved and consists of two α-helices and three β-strands arranged as β1-α1-β2-β3-α2. ClC CBS domain mutations cause muscle and bone disease and alter ClC gating. However, the precise functional roles of CBS domains and the structural bases by which they regulate ClC function are poorly understood. CLH-3a and CLH-3b are C. elegans ClC anion channel splice variants with strikingly different biophysical properties. Splice variation occurs at cytoplasmic N- and C-termini and includes several amino acids that form α2 of the second CBS domain (CBS2). We demonstrate that interchanging α2 between CLH-3a and CLH-3b interchanges their gating properties. The "R-helix" of ClC proteins forms part of the ion-conducting pore and selectivity filter and is connected to the cytoplasmic C-terminus via a short stretch of cytoplasmic amino acids termed the "R-helix linker". C-terminus conformation changes could cause R-helix structural rearrangements via this linker. X-ray structures of three ClC protein cytoplasmic C-termini suggest that α2 of CBS2 and the R-helix linker could be closely apposed and may therefore interact. We found that mutating apposing amino acids in α2 and the R-helix linker of CLH-3b was sufficient to give rise to CLH-3a-like gating. We postulate that the R-helix linker interacts with CBS2 α2, and that this putative interaction provides a pathway by which cytoplasmic C-terminus conformational changes induce conformational changes in membrane domains that in turn modulate ClC function.

Biochemical and functional studies on the regulation of the Saccharomyces cerevisiae AMPK homolog SNF1

AMP-activated protein kinase (AMPK) is a master metabolic regulator for controlling cellular energy homeostasis. Its homolog in yeast, SNF1, is activated in response to glucose depletion and other stresses. The catalytic (alpha) subunit of AMPK/SNF1, Snf1 in yeast, contains a protein Ser/Thr kinase domain (KD), an auto-inhibitory domain (AID), and a region that mediates interactions with the two regulatory (beta and gamma) subunits. Previous studies suggested that Snf1 contains an additional segment, a regulatory sequence (RS, corresponding to residues 392-518), which may also have an important role in regulating the activity of the enzyme. The crystal structure of the heterotrimer core of Saccharomyces cerevisiae SNF1 showed interactions between a part of the RS (residues 460-498) and the gamma subunit Snf4. Here we report biochemical and functional studies on the regulation of SNF1 by the RS. GST pulldown experiments demonstrate strong and direct interactions between residues 450-500 of the RS and the heterotrimer core, and single-site mutations in the RS-Snf4 interface can greatly reduce these interactions in vitro. On the other hand, functional studies appear to show only small effects of the RS-Snf4 interactions on the activity of SNF1 in vivo. This suggests that residues 450-500 may be constitutively associated with Snf4, and the remaining segments of the RS, as well as the AID, may be involved in regulating SNF1 activity.

Escherichia coli expression, purification and characterization of functional full-length recombinant alpha2beta2gamma3 heterotrimeric complex of human AMP-activated protein kinase

AMP-activated protein kinase (AMPK) is an energy-sensing serine/threonine protein kinase that plays a central role in whole-body energy homeostasis. AMPK is a heterotrimeric enzyme with a catalytic (alpha) subunit and two regulatory (beta and gamma) subunits. The muscle-specific AMPK heterotrimeric complex (alpha2beta2gamma3) is involved in glucose and fat metabolism in skeletal muscle and therefore has emerged as an attractive target for drug development for diabetes and metabolic syndrome. To date, expression of recombinant full-length human AMPK alpha2beta2gamma3 has not been reported. Here we describe the expression, purification and biochemical characterization of functional full-length AMPK alpha2beta2gamma3 heterotrimeric complex using an Escherichia coli expression system. All three subunits of AMPK alpha2beta2gamma3 were transcribed as a single tricistronic transcript driven by the T7 RNA polymerase promoter, allowing spontaneous formation of the heterotrimeric complex in the bacterial cytosol. The self-assembled trimeric complex was purified from the cell lysate by nickel-ion chromatography using the hexahistidine tag fused exclusively at the N-terminus of the alpha 2 domain. The un-assembled beta 2 and gamma 3 domains were removed by extensive washing of the column. Further purification of the heterotrimer was performed using size exclusion chromatography. The final yield of the recombinant AMPK alpha2beta2gamma3 complex was 1.1mg/L culture in shaker flasks. The E. coli expressed enzyme was catalytically inactive after purification, but was activated in vitro by upstream kinases such as CaMKKbeta and LKB1. The kinase activity of activated AMPK alpha2beta2gamma3 complex was significantly enhanced by AMP (an allosteric activator) but not by thienopyridone A-769662, a known small molecule activator of AMPK. Mass spectrometric characterization of recombinant AMPK alpha2beta2gamma3 showed significant heterogeneity before and after activation that could potentially hamper crystallographic studies of this complex.

Binding of S-methyl-5'-thioadenosine and S-adenosyl-L-methionine to protein MJ0100 triggers an open-to-closed conformational change in its CBS motif pair

Cystathionine beta-synthase (CBS) domains are small motifs that are present in proteins with completely different functions. Several genetic diseases in humans have been associated with mutations in their sequence, which has made them promising targets for rational drug design. The protein MJ0100 from Methanocaldococcus jannaschii includes a DUF39 domain of so far unknown function and a CBS domain pair (Bateman domain) at its C-terminus. This work presents the crystallographic analysis of four different states of the CBS motif pair of MJ0100 in complex with different numbers of S-adenosyl-L-methionine (SAM) and S-methyl-5'-thioadenosine (MTA) ligands, providing evidence that ligand-induced conformational reorganization of Bateman domain dimers could be an important regulatory mechanism. These observations are in contrast to what is known from most of the other Bateman domain structures but are supported by recent studies on the magnesium transporter MgtE. Our structures represent the first example of a CBS domain protein complexed with SAM and/or MTA and might provide a structural basis for understanding the molecular mechanisms regulated by SAM upon binding to the C-terminal domain of human CBS, whose structure remains unknown.

Purification, crystallization and preliminary crystallographic analysis of protein MJ1225 from Methanocaldococcus jannaschii, a putative archaeal homologue of gamma-AMPK

In mammals, AMP-activated protein kinase (AMPK) is a heterotrimeric protein composed of a catalytic serine/threonine kinase subunit (alpha) and two regulatory subunits (beta and gamma). The gamma subunit senses the intracellular energy status by competitively binding AMP and ATP and is thought to be responsible for allosteric regulation of the whole complex. This work describes the purification and preliminary crystallographic analysis of protein MJ1225 from Methanocaldococcus jannaschii, an archaeal homologue of gamma-AMPK. The purified protein was crystallized using the hanging-drop vapour-diffusion method. Diffraction data for MJ1225 were collected to 2.3 A resolution using synchrotron radiation. The crystals belonged to space group H32, with unit-cell parameters a = b = 108.95, c = 148.08 A, alpha = beta = 90.00, gamma = 120.00 degrees . Preliminary analysis of the X-ray data indicated that there was one molecule per asymmetric unit.

Maize AKINbetagamma dimerizes through the KIS/CBM domain and assembles into SnRK1 complexes

The SNF1/AMPK/SnRK1 complex is an intracellular energy sensor composed of three types of subunits: the SnRK1 kinase and two regulatory, non-catalytic subunits (designated beta and gamma). We have previously described an atypical plant gamma-subunit, AKINbetagamma, which contains an N-terminal tail similar to the so-called KIS domain normally present in beta-subunits. However, it is not known whether AKINbetagamma normally associates with endogenous SnRK1 complexes in vivo, nor how its unique domain structure might contribute to SnRK1 function. Here, we present evidence that maize AKINbetagamma is an integral component of active SnRK1 complexes in plant cells. Using complementary methodological approaches, we also show that AKINbetagamma associates through homomeric interactions mediated by both, the gamma- and, unexpectedly, the KIS/CBM domain.

Purification, crystallization and preliminary X-ray diffraction analysis of the CBS-domain pair from the Methanococcus jannaschii protein MJ0100

CBS domains are small protein motifs consisting of a three-stranded beta-sheet and two alpha-helices that are present in proteins of all kingdoms of life and in proteins with completely different functions. Several genetic diseases in humans have been associated with mutations in their sequence, which has made them promising targets for rational drug design. The C-terminal domain of the Methanococcus jannaschii protein MJ0100 includes a CBS-domain pair and has been overexpressed, purified and crystallized. Crystals of selenomethionine-substituted (SeMet) protein were also grown. The space group of both the native and SeMet crystals was determined to be orthorhombic P2(1)2(1)2(1), with unit-cell parameters a = 80.9, b = 119.5, c = 173.3 A. Preliminary analysis of the X-ray data indicated that there were eight molecules per asymmetric unit in both cases.

Purification, crystallization and preliminary X-ray diffraction analysis of a cystathionine beta-synthase domain-containing protein, CDCP2, from Arabidopsis thaliana

Cystathione beta-synthase domain-containing protein 2 (CDCP2) from Arabidopsis thaliana has been overexpressed and purified to homogeneity. As an initial step towards three-dimensional structure determination, crystals of recombinant CDCP2 protein have been obtained using polyethylene glycol 8000 as a precipitant. The crystals diffracted to 2.4 A resolution using synchrotron radiation and belonged to the trigonal space group P3(1)21 or P3(2)21, with unit-cell parameters a = b = 56.360, c = 82.596 A, alpha = beta = 90, gamma = 120 degrees . The asymmetric unit contains one CDCP2 molecule and the solvent content is approximately 41%.

Roles of the glycogen-binding domain and Snf4 in glucose inhibition of SNF1 protein kinase

The SNF1/AMP-activated protein kinase (AMPK) family is required for adaptation to metabolic stress and energy homeostasis. The gamma subunit of AMPK binds AMP and ATP, and mutations that affect binding cause human disease. We have here addressed the role of the Snf4 (gamma) subunit in regulating SNF1 protein kinase in response to glucose availability in Saccharomyces cerevisiae. Previous studies of mutant cells lacking Snf4 suggested that Snf4 counteracts autoinhibition by the C-terminal sequence of the Snf1 catalytic subunit but is dispensable for glucose regulation, and AMP does not activate SNF1 in vitro. We first introduced substitutions at sites that, in AMPK, contribute to nucleotide binding and regulation. Mutations at several sites relieved glucose inhibition of SNF1, as judged by catalytic activity, phosphorylation of the activation-loop Thr-210, and growth assays, although analogs of the severe human mutations R531G/Q had little effect. We further showed that alterations of Snf4 residues that interact with the glycogen-binding domain (GBD) of the beta subunit strongly relieved glucose inhibition. Finally, substitutions in the GBD of the Gal83 beta subunit that are predicted to disrupt interactions with Snf4 and also complete deletion of the GBD similarly relieved glucose inhibition of SNF1. Analysis of mutant cells lacking glycogen synthase showed that regulation of SNF1 is normal in the absence of glycogen. These findings reveal novel roles for Snf4 and the GBD in regulation of SNF1.

Structural properties of AMP-activated protein kinase: dimerization, molecular shape, and changes upon ligand binding

Heterotrimeric AMP-activated protein kinase (AMPK) is crucial for energy homeostasis of eukaryotic cells and organisms. Here we report on (i) bacterial expression of untagged mammalian AMPK isoform combinations, all containing gamma(1), (ii) an automated four-dimensional purification protocol, and (iii) biophysical characterization of AMPK heterotrimers by small angle x-ray scattering in solution (SAXS), transmission and scanning transmission electron microscopy (TEM, STEM), and mass spectrometry (MS). AMPK in solution at low concentrations (~1 mg/ml) largely consisted of individual heterotrimers in TEM analysis, revealed a precise 1:1:1 stoichiometry of the three subunits in MS, and behaved as an ideal solution in SAXS. At higher AMPK concentrations, SAXS revealed concentration-dependent, reversible dimerization of AMPK heterotrimers and formation of higher oligomers, also confirmed by STEM mass measurements. Single particle reconstruction and averaging by SAXS and TEM, respectively, revealed similar elongated, flat AMPK particles with protrusions and an indentation. In the lower AMPK concentration range, addition of AMP resulted in a significant decrease of the radius of gyration by approximately 5% in SAXS, which indicates a conformational switch in AMPK induced by ligand binding. We propose a structural model involving a ligand-induced relative movement of the kinase domain resulting in a more compact heterotrimer and a conformational change in the kinase domain that protects AMPK from dephosphorylation of Thr(172), thus positively affecting AMPK activity.

5'-AMP-activated protein kinase signaling in Caenorhabditis elegans

5'-AMP-activated protein kinase (AMPK) has been called "the metabolic master switch" because of its central role in regulating fuel homeostasis. AMPK, a heterotrimeric serine/threonine protein kinase composed of alpha, beta, and gamma subunits, is activated by upstream kinases and by 5'-AMP in response to various nutritional and stress signals. Downstream effects include regulation of metabolism, protein synthesis, cell growth, and mediation of the actions of a number of hormones, including leptin. However, AMPK research represents a young and growing field; hence, there are many unanswered questions regarding the control and action of AMPK. This review presents evidence for the existence of AMPK signaling pathways in Caenorhabditis elegans, a genetically tractable model organism that has yet to be fully exploited to elucidate AMPK signaling mechanisms.

Biochemical and structural characterization of a novel family of cystathionine beta-synthase domain proteins fused to a Zn ribbon-like domain

We have identified a novel family of proteins, in which the N-terminal cystathionine beta-synthase (CBS) domain is fused to the C-terminal Zn ribbon domain. Four proteins were overexpressed in Escherichia coli and purified: TA0289 from Thermoplasma acidophilum, TV1335 from Thermoplasma volcanium, PF1953 from Pyrococcus furiosus, and PH0267 from Pyrococcus horikoshii. The purified proteins had a red/purple color in solution and an absorption spectrum typical of rubredoxins (Rds). Metal analysis of purified proteins revealed the presence of several metals, with iron and zinc being the most abundant metals (2-67% of iron and 12-74% of zinc). Crystal structures of both mercury- and iron-bound TA0289 (1.5-2.0 A resolution) revealed a dimeric protein whose intersubunit contacts are formed exclusively by the alpha-helices of two cystathionine beta-synthase subdomains, whereas the C-terminal domain has a classical Zn ribbon planar architecture. All proteins were reversibly reduced by chemical reductants (ascorbate or dithionite) or by the general Rd reductase NorW from E. coli in the presence of NADH. Reduced TA0289 was found to be capable of transferring electrons to cytochrome C from horse heart. Likewise, the purified Zn ribbon protein KTI11 from Saccharomyces cerevisiae had a purple color in solution and an Rd-like absorption spectrum, contained both iron and zinc, and was reduced by the Rd reductase NorW from E. coli. Thus, recombinant Zn ribbon domains from archaea and yeast demonstrate an Rd-like electron carrier activity in vitro. We suggest that, in vivo, some Zn ribbon domains might also bind iron and therefore possess an electron carrier activity, adding another physiological role to this large family of important proteins.

Crystal structure of the heterotrimer core of Saccharomyces cerevisiae AMPK homologue SNF1

AMP-activated protein kinase (AMPK) is a central regulator of energy homeostasis in mammals and is an attractive target for drug discovery against diabetes, obesity and other diseases. The AMPK homologue in Saccharomyces cerevisiae, known as SNF1, is essential for responses to glucose starvation as well as for other cellular processes, although SNF1 seems to be activated by a ligand other than AMP. Here we report the crystal structure at 2.6 A resolution of the heterotrimer core of SNF1. The ligand-binding site in the gamma-subunit (Snf4) has clear structural differences from that of the Schizosaccharomyces pombe enzyme, although our crystallographic data indicate that AMP can also bind to Snf4. The glycogen-binding domain in the beta-subunit (Sip2) interacts with Snf4 in the heterotrimer but should still be able to bind carbohydrates. Our structure is supported by a large body of biochemical and genetic data on this complex. Most significantly, the structure reveals that part of the regulatory sequence in the alpha-subunit (Snf1) is sequestered by Snf4, demonstrating a direct interaction between the alpha- and gamma-subunits and indicating that our structure may represent the heterotrimer core of SNF1 in its activated state.

The Structure of the Cytoplasmic Domain of the Chloride Channel ClC-Ka Reveals a Conserved Interaction Interface

The cytoplasmic domains of ClC chloride channels and transporters are ubiquitously found in eukaryotic family members and have been suggested to be involved in the regulation of ion transport. All cytoplasmic ClC domains share a conserved scaffold that contains a pair of CBS motifs. Here we describe the structure of the cytoplasmic component of the human chloride channel ClC-Ka at 1.6 A resolution. The structure reveals a dimeric organization of the domain that is unusual for CBS motif containing proteins. Using a biochemical approach combining mutagenesis, crosslinking, and analytical ultracentrifugation, we demonstrate that the interaction interface is preserved in solution and that the distantly related channel ClC-0 likely exhibits a similar structural organization. Our results reveal a conserved interaction interface that relates the cytoplasmic domains of ClC proteins and establish a structural relationship that is likely general for this important family of transport proteins.