Interplay of Structural Disorder and Short Binding Elements in the Cellular Chaperone Function of Plant Dehydrin ERD14.

Details of the functional mechanisms of intrinsically disordered proteins (IDPs) in living cells is an area not frequently investigated. Here, we dissect the molecular mechanism of action of an IDP in cells by detailed structural analyses based on an in-cell nuclear magnetic resonance experiment. We show that the ID stress protein (IDSP) A. thaliana Early Response to Dehydration (ERD14) is capable of protecting E. coli cells under heat stress. The overexpression of ERD14 increases the viability of E. coli cells from 38.9% to 73.9% following heat stress (50 °C × 15 min). We also provide evidence that the protection is mainly achieved by protecting the proteome of the cells. In-cell NMR experiments performed in E. coli cells show that the protective activity is associated with a largely disordered structural state with conserved, short sequence motifs (K- and H-segments), which transiently sample helical conformations in vitro and engage in partner binding in vivo. Other regions of the protein, such as its S segment and its regions linking and flanking the binding motifs, remain unbound and disordered in the cell. Our data suggest that the cellular function of ERD14 is compatible with its residual structural disorder in vivo.

PP2A regulates autophagy in two alternative ways in Drosophila.

Protein phosphatase 2A (PP2A) holoenzyme is a heterotrimeric complex, consisting of A, B and C subunits. The catalytic subunit PP2A-C (microtubule star/mts) binds to the C-terminal part of the scaffold protein PP2A-A (PP2A-29B). In Drosophila, there are three different forms of B subunits (widerborst/wdb, twins/tws and PP2A-B'), which determine the subcellular localization and substrate specificity of the holoenzyme. Previous studies demonstrated that PP2A is involved in the control of TOR-dependent autophagy both in yeast and mammals. Furthermore, in Drosophila, wdb genetically interacts with the PtdIns3K/PTEN/Akt signaling cascade, which is a main upstream regulatory system of dTOR. Here we demonstrate that in Drosophila, two different PP2A complexes (containing B' or wdb subunit) play essential roles in the regulation of starvation-induced autophagy. The PP2A-A/wdb/C complex acts upstream of dTOR, whereas the PP2A-A/B'/C complex functions as a target of dTOR and may regulate the elongation of autophagosomes and their subsequent fusion with lysosomes. We also identified three Drosophila Atg orthologs (Atg14, Atg17 and Atg101), which represent potential targets of the PP2A-A/B'/C complex during autophagy.

Plasma membrane H(+) -ATPase gene expression, protein level and activity in growing and non-growing regions of barley (Hordeum vulgare) leaves.

Plasma membrane proton ATPase (PM-H⁺-ATPase) is the key means through which plant cells energize nutrient uptake and acidify the apoplast. Both of these processes aid cell elongation; yet, it is not known how such a suspected role of the PM-H⁺-ATPase in growth is reflected through changes in its transcript level and activity in grass leaves. In the present study on leaf three of barley, the elongation zone and the emerged blade, which contained fully expanded cells were analyzed. Plasma membranes were isolated and used to assay the activity (ATPase assay) and abundance (western blotting) of PM-H⁺-ATPase protein. Expression of mRNA was quantified using real-time polymerase chain reaction (qPCR). PM-H⁺-ATPase transcript and protein level and activity differed little between growing and non-growing leaf regions when values were related to unit extracted total RNA and cell number, respectively. However, when values were related to unit surface area of plasma membrane, they were more than twice as high in growing compared with non-growing leaf tissue. It is concluded that this higher surface density of PM-H⁺-ATPase activity in growing barley leaf tissue aids apoplast acidification and cell expansion.

A novel role for the Drosophila epsin (lqf): involvement in autophagy.

Screening P-element-induced mutant collections, 52 lines were selected as potentially defected ones in endocytosis or autophagy. After excluding those which were rescued by 20-hydroxyecdysone treatment, the exact position of the inserted P-element was determined in the remaining lines. In the case of l(3)S011027 stock, the liquid facets (lqf) gene was affected which codes an epsin-homolog protein in Drosophila. We reveal that Lqf is essential to the receptor-mediated endocytosis of larval serum proteins (LSPs) in the larval fat body cells of Drosophila. In l(3)S011027 line, lack of Lqf fails the formation of autophagosomes thus leading to the arrest of destroying of trophocytes. Transgenic larvae carrying Lqf-RNAi construct were unable to generate endocytic and autophagic vacuoles and led to a prolonged larval stage. On the other hand, Lqf protein showed an exclusive colocalization with the LysoTracker Red- or GFP-Atg8a labeled autophagosomes. By using the antiserum generated against the fifth exon of lqf, we demonstrated that prior to the onset of developmental autophagy the Lqf protein was present in the nucleus of fat body cell, but thereafter the protein was localized in the territory of endocytic and autophagic vacuoles. The fact that the inhibition of the target of rapamycin (TOR) did not restore the autophagic process and the normal development in the case of lqf mutant larvae points to that the Lqf is downstream to the TOR, the central kinase of the autophagy pathway.

SNF4Agamma, the Drosophila AMPK gamma subunit is required for regulation of developmental and stress-induced autophagy.

In holometabolous insects including Drosophila melanogaster a wave of autophagy triggered by 20-hydroxyecdysone is observed in the larval tissues during the third larval stage of metamorphosis. We used this model system to study the genetic regulation of autophagy. We performed a genetic screen to select P-element insertions that affect autophagy in the larval fat body. Light and electron microscopy of one of the isolated mutants (l(3)S005042) revealed the absence of autophagic vesicles in their fat body cells during the third larval stage. We show that formation of autophagic vesicles cannot be induced by 20-hydroxyecdysone in the tissues of mutant flies and represent evidence demonstrating that the failure to form autophagic vesicles is due to the insertion of a P-element into the gene coding SNF4Agamma, the Drosophila homologue of the AMPK (AMP-activated protein kinase) gamma subunit. The ability to form autophagic vesicles (wild-type phenotype) can be restored by remobilization of the P-element in the mutant. Silencing of SNF4Agamma by RNAi suppresses autophagic vesicle formation in wild-type flies. We raised an antibody against SNF4Agamma and showed that this gene product is constitutively present in the wild-type larval tissues during postembryonal development. SNF4Agamma is nearly absent from the cells of homozygous mutants. SNF4Agamma translocates into the nuclei of fat body cells at the onset of the wandering stage concurrently with the beginning of the autophagic process. Our results demonstrate that SNF4Agamma has an essential role in the regulation of autophagy in Drosophila larval fat body cells.

B lymphocytes and macrophages release cell membrane deposited C3-fragments on exosomes with T cell response-enhancing capacity.

Recently exosomes have been shown to play important roles in several immune phenomena. These small vesicles contain MHC proteins along with co-stimulatory and adhesion molecules, and mediate antigen presentation to T cells. In the present study we show that upon incubation with autologous serum, murine macrophages and B cells--but not T lymphocytes--fix C3-fragments covalently to the cell membrane and release them on exosomes in a time dependent fashion. While in the case of human B lymphocytes CR2 has been shown to serve as the main C3b-acceptor site, here we clearly demonstrate that cells derived from CR1/2 KO animals also have the capacity to fix C3b covalently. This finding points to a major difference between human and murine systems, and suggests the existence of additional acceptor sites on the cell membrane. Here we show that C3-fragment containing exosomes derived from OVA loaded antigen presenting cells induce a significantly elevated T cell response in the presence of suboptimal antigen stimulus. These data reveal a novel function of cell surface-deposited C3-fragments and provide further evidence for the role of exosomes secreted by antigen presenting cells. Since fixation of C3b to plasma membranes can be substantial in the presence of pathogens; moreover tumor cells are also known to activate the complement system resulting in complement-deposition, C3-carrying exosomes released by these cells may play an important immunomodulatory role in vivo, as well.

Longevity pathways converge on autophagy genes to regulate life span in Caenorhabditis elegans.

Aging is a multifactorial process with many mechanisms contributing to the decline. Mutations decreasing insulin/IGF-1 (insulin-like growth factor-1) or TOR (target of rapamycin) kinase-mediated signaling, mitochondrial activity and food intake each extend life span in divergent animal phyla. Understanding how these genetically distinct mechanisms interact to control longevity is a fundamental and fascinating problem in biology. Here we show that mutational inactivation of autophagy genes, which are involved in the degradation of aberrant, damaged cytoplasmic constituents accumulating in all aging cells, accelerates the rate at which the tissues age in the nematode Caenorhabditis elegans. According to our results Drosophila flies deficient in autophagy are also short-lived. We further demonstrate that reduced activity of autophagy genes suppresses life span extension in mutant nematodes with inherent dietary restriction, aberrant insulin/IGF-1 or TOR signaling, and lowered mitochondrial respiration. These findings suggest that the autophagy gene cascade functions downstream of and is inhibited by different longevity pathways in C. elegans, therefore, their effects converge on autophagy genes to slow down aging and lengthen life span. Thus, autophagy may act as a central regulatory mechanism of animal aging.

The Drosophila homolog of Aut1 is essential for autophagy and development.

The Drosophila homolog of yeast Aut1, CG6877/Draut1, is a ubiquitously expressed cytosolic protein. Draut1 loss of function was achieved by expression of an inverted repeat transgene inducing RNA interference. The effect is temperature-dependent and resembles an allelic series as described by Fortier, E. and Belote, J.M. (Genesis 26 (2000) 240-244). Draut1 loss of function larvae are unable to induce autophagy and heterophagy in fat body cells before pupariation and die during metamorphosis. To our knowledge, this is the first report of a multicellular animal lacking the function of a gene participating in the protein conjugation systems of autophagy.