A new negative control gene for amino acid biosynthesis in Saccharomyces cerevisiae

Plasmodium falciparum cysteine protease Falcipain 3: A potential enzyme for proteolytic processing of histone acetyltransferase PfGCN5

In spite of 150 years of studying malaria, the unique features of the malarial parasite, Plasmodium, still perplex researchers. One of the methods by which the parasite manages its gene expression is epigenetic regulation, the champion of which is PfGCN5, an essential enzyme responsible for acetylating histone proteins. PfGCN5 is a ∼170 kDa chromatin-remodeling enzyme that harbors the conserved bromodomain and acetyltransferase domain situated in its C-terminus domain. Although the PfGCN5 proteolytic processing is essential for its activity, the specific protease involved in this process still remains elusive. Identification of PfGCN5 interacting proteins through immunoprecipitation (IP) followed by LC-tandem mass spectrometry analysis revealed the presence of food vacuolar proteins, such as the cysteine protease Falcipain 3 (FP3), in addition to the typical members of the PfGCN5 complex. The direct interaction between FP3 and PfGCN5 was further validated by in vitro pull-down assay as well as IP assay. Subsequently, use of cysteine protease inhibitor E64d led to the inhibition of protease-specific processing of PfGCN5 with concomitant enrichment and co-localization of PfGCN5 and FP3 around the food vacuole as evidenced by confocal microscopy as well as electron microscopy. Remarkably, the proteolytic cleavage of the nuclear protein PfGCN5 by food vacuolar protease FP3 is exceptional and atypical in eukaryotic organisms. Targeting the proteolytic processing of GCN5 and the associated protease FP3 could provide a novel approach for drug development aimed at addressing the growing resistance of parasites to current antimalarial drugs.

Plasmodium falciparum GCN5 acetyltransferase follows a novel proteolytic processing pathway that is essential for its function

The pathogenesis of human malarial parasite Plasmodium falciparum is interlinked with its timely control of gene expression during its complex life cycle. In this organism, gene expression is partially controlled through epigenetic mechanisms, the regulation of which is, hence, of paramount importance to the parasite. The P. falciparum (Pf)-GCN5 histone acetyltransferase (HAT), an essential enzyme, acetylates histone 3 and regulates global gene expression in the parasite. Here, we show the existence of a novel proteolytic processing for PfGCN5 that is crucial for its activity in vivo We find that a cysteine protease-like enzyme is required for the processing of PfGCN5 protein. Immunofluorescence and immuno-electron microscopy analysis suggest that the processing event occurs in the vicinity of the digestive vacuole of the parasite following its trafficking through the classical ER-Golgi secretory pathway, before it subsequently reaches the nucleus. Furthermore, blocking of PfGCN5 processing leads to the concomitant reduction of its occupancy at the gene promoters and a reduced H3K9 acetylation level at these promoters, highlighting the important correlation between the processing event and PfGCN5 activity. Altogether, our study reveals a unique processing event for a nuclear protein PfGCN5 with unforeseen role of a food vacuolar cysteine protease. This leads to a possibility of the development of new antimalarials against these targets.This article has an associated First Person interview with the first author of the paper.

The Histone Acetyltransferase Gcn5 Positively Regulates T Cell Activation

Histone acetyltransferases (HATs) regulate inducible transcription in multiple cellular processes and during inflammatory and immune response. However, the functions of general control nonrepressed-protein 5 (Gcn5), an evolutionarily conserved HAT from yeast to human, in immune regulation remain unappreciated. In this study, we conditionally deleted Gcn5 (encoded by the Kat2a gene) specifically in T lymphocytes by crossing floxed Gcn5 and Lck-Cre mice, and demonstrated that Gcn5 plays important roles in multiple stages of T cell functions including development, clonal expansion, and differentiation. Loss of Gcn5 functions impaired T cell proliferation, IL-2 production, and Th1/Th17, but not Th2 and regulatory T cell differentiation. Gcn5 is recruited onto the il-2 promoter by interacting with the NFAT in T cells upon TCR stimulation. Interestingly, instead of directly acetylating NFAT, Gcn5 catalyzes histone H3 lysine H9 acetylation to promote IL-2 production. T cell-specific suppression of Gcn5 partially protected mice from myelin oligodendrocyte glycoprotein-induced experimental autoimmune encephalomyelitis, an experimental model for human multiple sclerosis. Our study reveals previously unknown physiological functions for Gcn5 and a molecular mechanism underlying these functions in regulating T cell immunity. Hence Gcn5 may be an important new target for autoimmune disease therapy.

The Histone Acetyltransferase GCN5 Expression Is Elevated and Regulated by c-Myc and E2F1 Transcription Factors in Human Colon Cancer

The histone acetyltransferase GCN5 has been suggested to be involved in promoting cancer cell growth. But its role in human colon cancer development remains unknown. Herein we discovered that GCN5 expression is significantly upregulated in human colon adenocarcinoma tissues. We further demonstrate that GCN5 is upregulated in human colon cancer at the mRNA level. Surprisingly, two transcription factors, the oncogenic c-Myc and the proapoptotic E2F1, are responsible for GCN5 mRNA transcription. Knockdown of c-Myc inhibited colon cancer cell proliferation largely through downregulating GCN5 transcription, which can be fully rescued by the ectopic GCN5 expression. In contrast, E2F1 expression induced human colon cancer cell death, and suppression of GCN5 expression in cells with E2F1 overexpression further facilitated cell apoptosis, suggesting that GCN5 expression is induced by E2F1 as a possible negative feedback in suppressing E2F1-mediated cell apoptosis. In addition, suppression of GCN5 with its specific inhibitor CPTH2 inhibited human colon cancer cell growth. Our studies reveal that GCN5 plays a positive role in human colon cancer development, and its suppression holds a great therapeutic potential in antitumor therapy.

Negative regulatory gene for general control of amino acid biosynthesis in Saccharomyces cerevisiae

In Saccharomyces cerevisiae, many amino acid biosynthetic pathways are coregulated by a complex general control system: starvation for a single amino acid results in the derepression of amino acid biosynthetic genes in multiple pathways. Derepression of these genes is mediated by positive (GCN) and negative (GCD) regulatory genes. In this paper we describe the isolation and characterization of a previously unreported negative regulatory gene, GCD3. A gcd3 mutation is recessive to wild type, confers resistance to multiple amino acid analogs, and results in overproduction and partially constitutive elevation of mRNA levels for amino acid biosynthetic genes. Furthermore, a gcd3 mutation can overcome the derepression-deficient phenotype of mutations in the positive regulatory GCN1, GCN2, and GCN3 genes. However, the gcd3 mutation cannot overcome the derepression-deficient phenotype of a gcn4 mutation, suggesting that GCD3 acts as a negative regulator of the important GCN4 gene. Northern blot analysis confirmed this conclusion, in that the steady-state levels of GCN4 mRNA are greatly increased in a gcd3 mutant. Thus, the negative regulatory gene GCD3 plays a central role in derepression of amino acid biosynthetic genes.

Negative regulatory gene for general control of amino acid biosynthesis in Saccharomyces cerevisiae

In Saccharomyces cerevisiae, many amino acid biosynthetic pathways are coregulated by a complex general control system: starvation for a single amino acid results in the derepression of amino acid biosynthetic genes in multiple pathways. Derepression of these genes is mediated by positive (GCN) and negative (GCD) regulatory genes. In this paper we describe the isolation and characterization of a previously unreported negative regulatory gene, GCD3. A gcd3 mutation is recessive to wild type, confers resistance to multiple amino acid analogs, and results in overproduction and partially constitutive elevation of mRNA levels for amino acid biosynthetic genes. Furthermore, a gcd3 mutation can overcome the derepression-deficient phenotype of mutations in the positive regulatory GCN1, GCN2, and GCN3 genes. However, the gcd3 mutation cannot overcome the derepression-deficient phenotype of a gcn4 mutation, suggesting that GCD3 acts as a negative regulator of the important GCN4 gene. Northern blot analysis confirmed this conclusion, in that the steady-state levels of GCN4 mRNA are greatly increased in a gcd3 mutant. Thus, the negative regulatory gene GCD3 plays a central role in derepression of amino acid biosynthetic genes.