Interaction of tau with HNRNPA2B1 and N6-methyladenosine RNA mediates the progression of tauopathy.

The microtubule-associated protein tau oligomerizes, but the actions of oligomeric tau (oTau) are unknown. We have used Cry2-based optogenetics to induce tau oligomers (oTau-c). Optical induction of oTau-c elicits tau phosphorylation, aggregation, and a translational stress response that includes stress granules and reduced protein synthesis. Proteomic analysis identifies HNRNPA2B1 as a principle target of oTau-c. The association of HNRNPA2B1 with endogenous oTau was verified in neurons, animal models, and human Alzheimer brain tissues. Mechanistic studies demonstrate that HNRNPA2B1 functions as a linker, connecting oTau with N6-methyladenosine (m6A) modified RNA transcripts. Knockdown of HNRNPA2B1 prevents oTau or oTau-c from associating with m6A or from reducing protein synthesis and reduces oTau-induced neurodegeneration. Levels of m6A and the m6A-oTau-HNRNPA2B1 complex are increased up to 5-fold in the brains of Alzheimer subjects and P301S tau mice. These results reveal a complex containing oTau, HNRNPA2B1, and m6A that contributes to the integrated stress response of oTau.

Frontotemporal Dementia P301L Mutation Potentiates but Is Not Sufficient to Cause the Formation of Cytotoxic Fibrils of Tau.

The P301L mutation in tau protein is a prevalent pathogenic mutation associated with neurodegenerative frontotemporal dementia, FTD. The mechanism by which P301L triggers or facilitates neurodegeneration at the molecular level remains unclear. In this work, we examined the effect of the P301L mutation on the biochemical and biological characteristics of pathologically relevant hyperphosphorylated tau. Hyperphosphorylated P301L tau forms cytotoxic aggregates more efficiently than hyperphosphorylated wildtype tau or unphosphorylated P301L tau in vitro. Mechanistic studies establish that hyperphosphorylated P301L tau exacerbates endoplasmic reticulum (ER) stress-associated gene upregulation in a neuroblastoma cell line when compared to wildtype hyperphosphorylated tau treatment. Furthermore, the microtubule cytoskeleton is severely disrupted following hyperphosphorylated P301L tau treatment. A hyperphosphorylated tau aggregation inhibitor, apomorphine, also inhibits the harmful effects caused by P301L hyperphosphorylated tau. In short, the P301L single mutation within the core repeat domain of tau renders the underlying hyperphosphorylated tau more potent in eliciting ER stress and cytoskeleton damage. However, the P301L mutation alone, without hyperphosphorylation, is not sufficient to cause these phenotypes. Understanding the conditions and mechanisms whereby selective mutations aggravate the pathogenic activities of tau can provide pivotal clues on novel strategies for drug development for frontotemporal dementia and other related neurodegenerative tauopathies, including Alzheimer's disease.

Anti-fibrillization effects of sulfonamide derivatives on α-synuclein and hyperphosphorylated tau isoform 1N4R.

In contrast to Aß plaques, the spatiotemporal distribution of neurofibrillary tangles of hyperphosphorylated tau (p-tau) predicts cognitive impairment in Alzheimer's disease (AD), underscoring the key pathological role of p-tau and the utmost need to develop AD therapeutics centering upon the control of p-tau aggregation and cytotoxicity. Our drug discovery program is focused on compounds that prevent the aggregation and cytotoxicity of p-tau moieties of the tau isoform 1N4R due to its prevalence (1 N) and long-distance trans-synaptic propagation (4R). We prepared and tested twenty-four newly synthesized small molecules representing the urea (1, 2, 3), sulfonylurea (4), and sulfonamide (5-24) series and evaluated their anti-aggregation effects with biophysical methods (thioflavin T and S fluorescence assays, transmission electron microscopy) and intracellular inclusion cell-based assays. Pre-evaluation was performed on alpha-synuclein (α-syn) to identify molecules to be challenged with p-tau. The sulfonamide derivatives 18 and 20 exhibited an anti-fribrillization activity on α-syn and p-tau. Sulfonamide compounds 18 and 20 reduced inclusion formation in M17D neuroblastoma cells that express inclusion-prone αSynuclein3K::YFP. This project advances new concepts in targeting prone-to-aggregate proteins such as α-syn and p-tau, and provides a molecular scaffold for further optimization and pre-clinical studies focused on AD drug development.

Correction: Genome, Functional Gene Annotation, and Nuclear Transformation of the Heterokont Oleaginous Alga Nannochloropsis oceanica CCMP1779.

[This corrects the article DOI: 10.1371/journal.pgen.1003064.].

An Energy-Independent Pro-longevity Function of Triacylglycerol in Yeast.

Intracellular triacylglycerol (TAG) is a ubiquitous energy storage lipid also involved in lipid homeostasis and signaling. Comparatively, little is known about TAG's role in other cellular functions. Here we show a pro-longevity function of TAG in the budding yeast Saccharomyces cerevisiae. In yeast strains derived from natural and laboratory environments a correlation between high levels of TAG and longer chronological lifespan was observed. Increased TAG abundance through the deletion of TAG lipases prolonged chronological lifespan of laboratory strains, while diminishing TAG biosynthesis shortened lifespan without apparently affecting vegetative growth. TAG-mediated lifespan extension was independent of several other known stress response factors involved in chronological aging. Because both lifespan regulation and TAG metabolism are conserved, this cellular pro-longevity function of TAG may extend to other organisms.

Critical roles of Shugoshin and histones as tension sensors during mitosis.

Biorientation of paired sister chromosomes is required to maintain mitotic fidelity. A critical signal indicative of bipolar attachment is tension between cohesion-linked sister chromatids. Key components of the tension signaling apparatus include the Shugoshin family of proteins and the tension sensing motif of histone H3. Shugoshin proteins are recruited to chromatin to create discrete domains integral to tension sensing. Many factors involved in the chromatin association of Shugoshin proteins are well established, most strikingly through modifications found directly on centromeric and pericentric chromatin. It has been well established that phosphorylation at the centromere is essential to nucleating Shugoshin recruitment, but recent evidence revealed a role for pericentric histones and acetylation in modulating Shugoshin recruitment and activity. These data demonstrate that chromatins are not simply passive cargo during mitosis, but are instead actively involved in their segregation.

The slim, the fat, and the obese: guess who lives the longest?

In a modern society that is increasingly older and "heavier," it is understandable that the majority favors a slimmer body that helps to sail smoothly into the dusk of life. Given the association between obesity and many metabolic and cardiovascular disorders, there are stern criticisms over such a thought of "good fat". Ironically, a phenomenon called "obesity paradox", that is, the overweight population purportedly enjoys the lowest all-cause mortality, and baffles open-minded clinicians and scientists. Lipids are essential to all life forms. Fat, in particular, triacylglycerol, also exists in different forms and in different locations in the human body, making any simple statement that vilifies all fat invalid. Whether the phenomenon of obesity paradox, indeed, has its root in a hitherto unrealized pro-survival function of fat deserves a serious look. Indeed, a recent publication using yeast as the model showed that elevation in the cellular storage of triacylglycerol extends lifespan in an energy expenditure independent fashion. In stark contrast, lean cells devoid of triacylglycerol biosynthetic capability die upon entering the senescence phase. Together, a new cytoprotective function of fat emerges. This mini-review aims to discuss potential mechanisms for the observed lifespan preservation function of triacylglycerol.

Protein interaction module-assisted function X (PIMAX) approach to producing challenging proteins including hyperphosphorylated tau and active CDK5/p25 kinase complex.

Many biomedically critical proteins are underrepresented in proteomics and biochemical studies because of the difficulty of their production in Escherichia coli. These proteins might possess posttranslational modifications vital to their functions, tend to misfold and be partitioned into bacterial inclusion bodies, or act only in a stoichiometric dimeric complex. Successful production of these proteins requires efficient interaction between these proteins and a specific "facilitator," such as a protein-modifying enzyme, a molecular chaperone, or a natural physical partner within the dimeric complex. Here we report the design and application of a protein interaction module-assisted function X (PIMAX) system that effectively overcomes these hurdles. By fusing two proteins of interest to a pair of well-studied protein-protein interaction modules, we were able to potentiate the association of these two proteins, resulting in successful production of an enzymatically active cyclin-dependent kinase complex and hyperphosphorylated tau protein, which is intimately linked to Alzheimer disease. Furthermore, using tau isoforms quantitatively phosphorylated by GSK-3ß and CDK5 kinases via PIMAX, we demonstrated the hyperphosphorylation-stimulated tau oligomerization in vitro, paving the way for new Alzheimer disease drug discoveries. Vectors for PIMAX can be easily modified to meet the needs of different applications. This approach thus provides a convenient and modular suite with broad implications for proteomics and biomedical research.

A galactoglycerolipid lipase is required for triacylglycerol accumulation and survival following nitrogen deprivation in Chlamydomonas reinhardtii.

Following N deprivation, microalgae accumulate triacylglycerols (TAGs). To gain mechanistic insights into this phenomenon, we identified mutants with reduced TAG content following N deprivation in the model alga Chlamydomonas reinhardtii. In one of the mutants, the disruption of a galactoglycerolipid lipase-encoding gene, designated PLASTID GALACTOGLYCEROLIPID DEGRADATION1 (PGD1), was responsible for the primary phenotype: reduced TAG content, altered TAG composition, and reduced galactoglycerolipid turnover. The recombinant PGD1 protein, which was purified from Escherichia coli extracts, hydrolyzed monogalactosyldiacylglycerol into its lyso-lipid derivative. In vivo pulse-chase labeling identified galactoglycerolipid pools as a major source of fatty acids esterified in TAGs following N deprivation. Moreover, the fatty acid flux from plastid lipids to TAG was decreased in the pgd1 mutant. Apparently, de novo-synthesized fatty acids in Chlamydomonas reinhardtii are, at least partially, first incorporated into plastid lipids before they enter TAG synthesis. As a secondary effect, the pgd1 mutant exhibited a loss of viability following N deprivation, which could be avoided by blocking photosynthetic electron transport. Thus, the pgd1 mutant provides evidence for an important biological function of TAG synthesis following N deprivation, namely, relieving a detrimental overreduction of the photosynthetic electron transport chain.

Genome, functional gene annotation, and nuclear transformation of the heterokont oleaginous alga Nannochloropsis oceanica CCMP1779.

Unicellular marine algae have promise for providing sustainable and scalable biofuel feedstocks, although no single species has emerged as a preferred organism. Moreover, adequate molecular and genetic resources prerequisite for the rational engineering of marine algal feedstocks are lacking for most candidate species. Heterokonts of the genus Nannochloropsis naturally have high cellular oil content and are already in use for industrial production of high-value lipid products. First success in applying reverse genetics by targeted gene replacement makes Nannochloropsis oceanica an attractive model to investigate the cell and molecular biology and biochemistry of this fascinating organism group. Here we present the assembly of the 28.7 Mb genome of N. oceanica CCMP1779. RNA sequencing data from nitrogen-replete and nitrogen-depleted growth conditions support a total of 11,973 genes, of which in addition to automatic annotation some were manually inspected to predict the biochemical repertoire for this organism. Among others, more than 100 genes putatively related to lipid metabolism, 114 predicted transcription factors, and 109 transcriptional regulators were annotated. Comparison of the N. oceanica CCMP1779 gene repertoire with the recently published N. gaditana genome identified 2,649 genes likely specific to N. oceanica CCMP1779. Many of these N. oceanica-specific genes have putative orthologs in other species or are supported by transcriptional evidence. However, because similarity-based annotations are limited, functions of most of these species-specific genes remain unknown. Aside from the genome sequence and its analysis, protocols for the transformation of N. oceanica CCMP1779 are provided. The availability of genomic and transcriptomic data for Nannochloropsis oceanica CCMP1779, along with efficient transformation protocols, provides a blueprint for future detailed gene functional analysis and genetic engineering of Nannochloropsis species by a growing academic community focused on this genus.

Galectin-3 aggravates microglial activation and tau transmission in tauopathy.

Alzheimer's disease is characterized by the accumulation of amyloid-ß plaques, aggregation of hyperphosphorylated tau (pTau), and microglia activation. Galectin-3 (Gal3) is a ß-galactoside-binding protein that has been implicated in amyloid pathology. Its role in tauopathy remains enigmatic. Here, we showed that Gal3 was upregulated in the microglia of humans and mice with tauopathy. pTau triggered the release of Gal3 from human induced pluripotent stem cell-derived microglia in both its free and extracellular vesicular-associated (EV-associated) forms. Both forms of Gal3 increased the accumulation of pathogenic tau in recipient cells. Binding of Gal3 to pTau greatly enhanced tau fibrillation. Besides Gal3, pTau was sorted into EVs for transmission. Moreover, pTau markedly enhanced the number of EVs released by iMGL in a Gal3-dependent manner, suggesting a role of Gal3 in biogenesis of EVs. Single-cell RNA-Seq analysis of the hippocampus of a mouse model of tauopathy (THY-Tau22) revealed a group of pathogenic tau-evoked, Gal3-associated microglia with altered cellular machineries implicated in neurodegeneration, including enhanced immune and inflammatory responses. Genetic removal of Gal3 in THY-Tau22 mice suppressed microglia activation, reduced the level of pTau and synaptic loss in neurons, and rescued memory impairment. Collectively, Gal3 is a potential therapeutic target for tauopathy.

Phosphopeptide enrichment with TiO2-modified membranes and investigation of tau protein phosphorylation.

Selective enrichment of phosphopeptides prior to their analysis by mass spectrometry (MS) is vital for identifying protein phosphorylation sites involved in cellular regulation. This study describes modification of porous nylon substrates with TiO2 nanoparticles to create membranes that rapidly enrich phosphopeptides. Membranes with a 22-mm diameter bind 540 nmol of phosphoangiotensin and recover 70% of the phosphopeptides in mixtures with a 15-fold excess of nonphosphorylated proteins. Recovery is 90% for a pure phosphopeptide. Insertion of small membrane disks into HPLC fittings allows rapid enrichment from 5 mL of 1 fmol/µL phosphoprotein digests and concentration into small-volume (tens of microliters) eluates. The combination of membrane enrichment with tandem mass spectrometry reveals seven phosphorylation sites from in vivo phosphorylated tau (p-tau) protein, which is associated with Alzheimer's disease.

Rapid triacylglycerol turnover in Chlamydomonas reinhardtii requires a lipase with broad substrate specificity.

When deprived of nitrogen (N), the photosynthetic microalga Chlamydomonas reinhardtii accumulates large quantities of triacylglycerols (TAGs), making it a promising source of biofuel. Prominent transcriptional changes associated with the conditions leading to TAG accumulation have been found, suggesting that the key enzymes for TAG metabolism might be among those that fluctuate in their expression during TAG synthesis and breakdown. Using a Saccharomyces cerevisiae lipase null mutant strain for functional complementation, we identified the CrLIP1 gene from Chlamydomonas based on its ability to suppress the lipase deficiency-related phenotypes of the yeast mutant. In Chlamydomonas, an inverse correlation was found between the CrLIP1 transcript level and TAG abundance when Chlamydomonas cultures were reversibly deprived of N. The CrLIP1 protein expressed and purified from Escherichia coli exhibited lipolytic activity against diacylglycerol (DAG) and polar lipids. The lipase domain of CrLIP1 is most similar to two human DAG lipases, DAGLα and DAGLß. The involvement of CrLIP1 in Chlamydomonas TAG hydrolysis was corroborated by reducing the abundance of the CrLIP1 transcript with an artificial micro-RNA, which resulted in an apparent delay in TAG lipolysis when N was resupplied. Together, these data suggest that CrLIP1 facilitates TAG turnover in Chlamydomonas primarily by degrading the DAG presumably generated from TAG hydrolysis.

Hyperphosphorylated Tau Inflicts Intracellular Stress Responses that Are Mitigated by Apomorphine.

Abnormal phosphorylation of the microtubule-binding protein tau in the brain is a key pathological marker for Alzheimer's disease and additional neurodegenerative tauopathies. However, how hyperphosphorylated tau causes cellular dysfunction or death that underlies neurodegeneration remains an unsolved question critical for the understanding of disease mechanism and the design of efficacious drugs. Using a recombinant hyperphosphorylated tau protein (p-tau) synthesized by the PIMAX approach, we examined how cells responded to the cytotoxic tau and explored means to enhance cellular resistance to tau attack. Upon p-tau uptake, the intracellular calcium levels rose promptly. Gene expression analyses revealed that p-tau potently triggered endoplasmic reticulum (ER) stress, unfolded protein response (UPR), ER stress-associated apoptosis, and pro-inflammation in cells. Proteomics studies showed that p-tau diminished heme oxygenase-1 (HO-1), an ER stress-associated anti-inflammation and anti-oxidative stress regulator, while stimulated the accumulation of MIOS and other proteins. p-Tau-induced ER stress-associated apoptosis and pro-inflammation are ameliorated by apomorphine, a brain-permeable prescription drug widely used to treat Parkinson's disease symptoms, and by overexpression of HO-1. Our results reveal probable cellular functions targeted by hyperphosphorylated tau. Some of these dysfunctions and stress responses have been linked to neurodegeneration in Alzheimer's disease. The observations that the ill effects of p-tau can be mitigated by a small compound and by overexpressing HO-1 that is otherwise diminished in the treated cells inform new directions of Alzheimer's disease drug discovery.

Hyperphosphorylated tau Inflicts Intracellular Stress Responses That Are Mitigated by Apomorphine.

Background: Abnormal phosphorylation of the microtubule-binding protein tau in the brain is a key pathological marker for Alzheimer's disease and additional neurodegenerative tauopathies. However, how hyperphosphorylated tau causes cellular dysfunction or death that underlie neurodegeneration remains an unsolved question critical for the understanding of disease mechanism and the design of efficacious drugs. Methods: Using a recombinant hyperphosphorylated tau protein (p-tau) synthesized by the PIMAX approach, we examined how cells responded to the cytotoxic tau and explored means to enhance cellular resistance to tau attack. Results: Upon p-tau uptake, the intracellular calcium levels rose promptly. Gene expression analyses revealed that p-tau potently triggered endoplasmic reticulum (ER) stress, Unfolded Protein Response (UPR), ER stress-associated apoptosis, and pro-inflammation in cells. Proteomics studies showed that p-tau diminished heme oxygenase-1 (HO-1), an ER stress associated anti-inflammation and anti-oxidative stress regulator, while stimulated the accumulation of MIOS and other proteins. P-tau-induced ER stress-associated apoptosis and pro-inflammation are ameliorated by apomorphine, a brain-permeable prescription drug widely used to treat Parkinson's disease symptoms, and by overexpression of HO-1. Conclusion: Our results reveal probable cellular functions targeted by hyperphosphorylated tau. Some of these dysfunctions and stress responses have been linked to neurodegeneration in Alzheimer's disease. The observations that the ill effects of p-tau can be mitigated by a small compound and by overexpressing HO-1 that is otherwise diminished in the treated cells inform new directions of Alzheimer's disease drug discovery.

Single-domain antibody-based noninvasive in vivo imaging of α-synuclein or tau pathology.

Intracellular deposition of α-synuclein and tau are hallmarks of synucleinopathies and tauopathies, respectively. Recently, several dye-based imaging probes with selectivity for tau aggregates have been developed, but suitable imaging biomarkers for synucleinopathies are still unavailable. Detection of both of these aggregates early in the disease process may allow for prophylactic therapies before functional impairments have manifested, highlighting the importance of developing specific imaging probes for these lesions. In contrast to the ß sheet dyes, single-domain antibodies, found in camelids and a few other species, are highly specific, and their small size allows better brain entry and distribution than whole antibodies. Here, we have developed such imaging ligands via phage display libraries derived from llamas immunized with α-synuclein and tau preparations, respectively. These probes allow noninvasive and specific in vivo imaging of α-synuclein versus tau pathology in mice, with the brain signal correlating strongly with lesion burden. These small antibody derivatives have great potential for in vivo diagnosis of these diseases.

Evaluation of N- and O-Linked Indole Triazines for a Dual Effect on α-Synuclein and Tau Aggregation.

Alzheimer's disease (AD) is the most prevalent neurodegenerative disorder underlying dementia in the geriatric population. AD manifests by two pathological hallmarks: extracellular amyloid-ß (Aß) peptide-containing senile plaques and intraneuronal neurofibrillary tangles comprised of aggregated hyperphosphorylated tau protein (p-tau). However, more than half of AD cases also display the presence of aggregated α-synuclein (α-syn)-containing Lewy bodies. Conversely, Lewy bodies disorders have been reported to have concomitant Aß plaques and neurofibrillary tangles. Our drug discovery program focuses on the synthesis of multitarget-directed ligands to abrogate aberrant α-syn, tau (2N4R), and p-tau (1N4R) aggregation and to slow the progression of AD and related dementias. To this end, we synthesized 11 compounds with a triazine-linker and evaluated their effectiveness in reducing α-syn, tau isoform 2N4R, and p-tau isoform 1N4R aggregation. We utilized biophysical methods such as thioflavin T (ThT) fluorescence assays, transmission electron microscopy (TEM), photoinduced cross-linking of unmodified proteins (PICUP), and M17D intracellular inclusion cell-based assays to evaluate the antiaggregation properties and cellular protection of our best compounds. We also performed disaggregation assays with isolated Aß-plaques from human AD brains. Our results demonstrated that compound 10 was effective in reducing both oligomerization and fibril formation of α-syn and tau isoform 2N4R in a dose-dependent manner via ThT and PICUP assays. Compound 10 was also effective at reducing the formation of recombinant α-syn, tau 2N4R, and p-tau 1N4R fibrils by TEM. Compound 10 reduced the development of α-syn inclusions in M17D neuroblastoma cells and stopped the seeding of tau P301S using biosensor cells. Disaggregation experiments showed smaller Aß-plaques and less paired helical filaments with compound 10. Compound 10 may provide molecular scaffolds for further optimization and preclinical studies for neurodegenerative proteinopathies.

Single domain antibodies targeting pathological tau protein: Influence of four IgG subclasses on efficacy and toxicity.

BACKGROUND: Eleven tau immunoglobulin G (IgG) antibodies have entered clinical trials to treat tauopathies, including Alzheimer's disease, but it is unclear which IgG subclass/subtype has the ideal efficacy and safety profile. Only two subtypes, with or without effector function, have been examined in the clinic and not for the same tau antibody. The few preclinical studies on this topic have only compared two subtypes of one antibody each and have yielded conflicting results. METHODS: We selected two single domain antibodies (sdAbs) derived from a llama immunized with tau proteins and utilized them to generate an array of Fc-(sdAb)2 subclasses containing identical tau binding domains but differing Fc region. Unmodified sdAbs and their IgG subclasses were tested for efficacy in primary cultures and in vivo microdialysis using JNPL3 tauopathy mice. FINDINGS: Unmodified sdAbs were non-toxic, blocked tau toxicity and promoted tau clearance. However, the efficacy/safety profile of their Fc-(sdAb)2 subclasses varied greatly within and between sdAbs. For one of them, all its subtypes were non-toxic, only those with effector function cleared tau, and were more effective in vivo than unmodified sdAb. For the other sdAb, all its subtypes were toxic in tauopathy cultures but not in wild-type cells, suggesting that bivalent binding of its tau epitope stabilizes a toxic conformation of tau, with major implications for tau pathogenesis. Likewise, its subclasses were less effective than the unmodified sdAb in clearing tau in vivo. INTERPRETATION: These findings indicate that tau antibodies with effector function are safe and better at clearing pathological tau than effectorless antibodies, Furthermore, tau antibodies can provide a valuable insight into tau pathogenesis, and some may aggravate it. FUNDING: Funding for these studies was provided by the National Institute of Health (R01 AG032611, R01 NS077239, RF1 NS120488, R21 AG 069475, R21 AG 058282, T32AG052909), and the NYU Alzheimer's Disease Center Pilot Grant Program (via P30 AG008051).

Changes in transcript abundance in Chlamydomonas reinhardtii following nitrogen deprivation predict diversion of metabolism.

Like many microalgae, Chlamydomonas reinhardtii forms lipid droplets rich in triacylglycerols when nutrient deprived. To begin studying the mechanisms underlying this process, nitrogen (N) deprivation was used to induce triacylglycerol accumulation and changes in developmental programs such as gametogenesis. Comparative global analysis of transcripts under induced and noninduced conditions was applied as a first approach to studying molecular changes that promote or accompany triacylglycerol accumulation in cells encountering a new nutrient environment. Towards this goal, high-throughput sequencing technology was employed to generate large numbers of expressed sequence tags of eight biologically independent libraries, four for each condition, N replete and N deprived, allowing a statistically sound comparison of expression levels under the two tested conditions. As expected, N deprivation activated a subset of control genes involved in gametogenesis while down-regulating protein biosynthesis. Genes for components of photosynthesis were also down-regulated, with the exception of the PSBS gene. N deprivation led to a marked redirection of metabolism: the primary carbon source, acetate, was no longer converted to cell building blocks by the glyoxylate cycle and gluconeogenesis but funneled directly into fatty acid biosynthesis. Additional fatty acids may be produced by membrane remodeling, a process that is suggested by the changes observed in transcript abundance of putative lipase genes. Inferences on metabolism based on transcriptional analysis are indirect, but biochemical experiments supported some of these deductions. The data provided here represent a rich source for the exploration of the mechanism of oil accumulation in microalgae.

Functional connection between histone acetyltransferase Gcn5p and methyltransferase Hmt1p.

Histone acetylation and methylation are linked to a variety of nuclear activities, most notably transcriptional regulation. Both synergistic and antagonistic relationships between these two modifications have been reported in different systems. Here we show that the budding yeast histone H4 arginine 3 (R3) methyltransferase Hmt1p binds acetylated histones H3 and H4, and importantly, that acetylated H4 is a significantly better methylation substrate for Hmt1p. Kinetic studies show that acetylation at any of the four acetylatable lysine residues of histone H4 results in more efficient methylation. Among the four, K8 acetylation imposes the strongest effect on reducing K(M), consistent with the observed acetylation-stimulated interaction. In vivo, hmt1Delta cells rescue the transcriptional defect caused by GCN5 deletion, indicating that one of the functions of Gcn5p is to neutralize the negative effect of Hmt1p. Mutating either K8 or R3 to alanine causes similar growth defects in selective histone and gcn5 mutant background, suggesting that these two residues function in the same pathway for optimal vegetative growth. Together, these results reveal a functional connection between histone acetylation, methylation, and two of the responsible enzymes, Gcn5p and Hmt1p.