The Polycomb group protein MEDEA and the DNA methyltransferase MET1 interact to repress autonomous endosperm development in Arabidopsis

In flowering plants, double fertilization of the female gametes, the egg and the central cell, initiates seed development to give rise to a diploid embryo and the triploid endosperm. In the absence of fertilization, the FERTILIZATION‐INDEPENDENT SEED Polycomb Repressive Complex 2 (FIS‐PRC2) represses this developmental process by histone methylation of certain target genes. The FERTILIZATION‐INDEPENDENT SEED (FIS) class genes MEDEA (MEA) and FERTILIZATION‐INDEPENDENT ENDOSPERM (FIE) encode two of the core components of this complex. In addition, DNA methylation establishes and maintains the repression of gene activity, for instance via DNA METHYLTRANSFERASE1 (MET1), which maintains methylation of symmetric CpG residues. Here, we demonstrate that Arabidopsis MET1 interacts with MEA in vitro and in a yeast two‐hybrid assay, similar to the previously identified interaction of the mammalian homologues DNMT1 and EZH2. MET1 and MEA share overlapping expression patterns in reproductive tissues before and after fertilization, a prerequisite for an interaction in vivo. Importantly, a much higher percentage of central cells initiate endosperm development in the absence of fertilization in mea‐1/MEA; met1‐3/MET1 as compared to mea‐1/MEA mutant plants. In addition, DNA methylation at the PHERES1 and MEA loci, imprinted target genes of the FIS‐PRC2, was affected in the mea‐1 mutant compared with wild‐type embryos. In conclusion, our data suggest a mechanistic link between two major epigenetic pathways involved in histone and DNA methylation in plants by physical interaction of MET1 with the FIS‐PRC2 core component MEA. This concerted action is relevant for the repression of seed development in the absence of fertilization.     

Single-unit responses selective for whole faces in the human amygdala

The human amygdala is critical for social cognition from faces, as borne out by impairments in recognizing facial emotion following amygdala lesions [1] and differential activation of the amygdala by faces [2-5]. Single-unit recordings in the primate amygdala have documented responses selective for faces, their identity, or emotional expression [6, 7], yet how the amygdala represents face information remains unknown. Does it encode specific features of faces that are particularly critical for recognizing emotions (such as the eyes), or does it encode the whole face, a level of representation that might be the proximal substrate for subsequent social cognition? We investigated this question by recording from over 200 single neurons in the amygdalae of seven neurosurgical patients with implanted depth electrodes [8]. We found that approximately half of all neurons responded to faces or parts of faces. Approximately 20% of all neurons responded selectively only to the whole face. Although responding most to whole faces, these neurons paradoxically responded more when only a small part of the face was shown compared to when almost the entire face was shown. We suggest that the human amygdala plays a predominant role in representing global information about faces, possibly achieved through inhibition between individual facial features.     

Towards a molecular description of intermediate filament structure and assembly

Intermediate filaments (IFs) represent one of the prominent cytoskeletal elements of metazoan cells. Their constituent proteins are coded by a multigene family, whose members are expressed in complex patterns that are controlled by developmental programs of differentiation. Hence, IF proteins found in epidermis differ significantly from those in muscle or neuronal tissues. Due to their fibrous nature, which stems from a fairly conserved central alpha-helical coiled-coil rod domain, IF proteins have long resisted crystallization and thus determination of their atomic structure. Since they represent the primary structural elements that determine the shape of the nucleus and the cell more generally, a major challenge is to arrive at a more rational understanding of how their nanomechanical properties effect the stability and plasticity of cells and tissues. Here, we review recent structural results of the coiled-coil dimer, assembly intermediates and growing filaments that have been obtained by a hybrid methods approach involving a rigorous combination of X-ray crystallography, small angle X-ray scattering, cryo-electron tomography, computational analysis and molecular modeling.     

Ca2+ -dependent interaction of S100A1 with F1-ATPase leads to an increased ATP content in cardiomyocytes

S100A1, a Ca(2+)-sensing protein of the EF-hand family that is expressed predominantly in cardiac muscle, plays a pivotal role in cardiac contractility in vitro and in vivo. It has recently been demonstrated that by restoring Ca(2+) homeostasis, S100A1 was able to rescue contractile dysfunction in failing rat hearts. Myocardial contractility is regulated not only by Ca(2+) homeostasis but also by energy metabolism, in particular the production of ATP. Here, we report a novel interaction of S100A1 with mitochondrial F(1)-ATPase, which affects F(1)-ATPase activity and cellular ATP production. In particular, cardiomyocytes that overexpress S100A1 exhibited a higher ATP content than control cells, whereas knockdown of S100A1 expression decreased ATP levels. In pull-down experiments, we identified the alpha- and beta-chain of F(1)-ATPase to interact with S100A1 in a Ca(2+)-dependent manner. The interaction was confirmed by colocalization studies of S100A1 and F(1)-ATPase and the analysis of the S100A1-F(1)-ATPase complex by gel filtration chromatography. The functional impact of this association is highlighted by an S100A1-mediated increase of F(1)-ATPase activity. Consistently, ATP synthase activity is reduced in cardiomyocytes from S100A1 knockout mice. Our data indicate that S100A1 might play a key role in cardiac energy metabolism.     

Peripheral nerve demyelination caused by a mutant Rho GTPase guanine nucleotide exchange factor, frabin/FGD4

GTPases of the Rho subfamily are widely involved in the myelination of the vertebrate nervous system. Rho GTPase activity is temporally and spatially regulated by a set of specific guanine nucleotide exchange factors (GEFs). Here, we report that disruption of frabin/FGD4, a GEF for the Rho GTPase cell-division cycle 42 (Cdc42), causes peripheral nerve demyelination in patients with autosomal recessive Charcot-Marie-Tooth (CMT) neuropathy. These data, together with the ability of frabin to induce Cdc42-mediated cell-shape changes in transfected Schwann cells, suggest that Rho GTPase signaling is essential for proper myelination of the peripheral nervous system.     

Nanomechanical interactions of phenylalanine-glycine nucleoporins studied by single molecule force-volume spectroscopy

Phenylalanine-glycine (FG)-repeat nucleoporins (Nups) form the major components of the selective gating mechanism in the nuclear pore complex (NPC). Hence, a primary requirement is to understand how they vacillate between preventing the access of passively diffusing molecules and promoting the translocation of receptor-bound cargo into the NPC. To shed light on such behavior, we have studied the nanomechanical properties of a cysteine-modified FG-rich C-terminal domain of hNup153 (i.e., cNup153) and its interactions with importin-beta. This is carried out using single molecule force spectroscopy (SMFS) with the atomic force microscope (AFM). In the absence of importin-beta, cNup153 is highly flexible and can be reversibly stretched and relaxed without any change to its intrinsic entropic elasticity, indicating a lack of intra-FG interactions, i.e., natively unfolded. Importin-beta-modified AFM tips reveal complex binding topologies with cNup153, and provide evidence for binding promiscuity in FG-receptor interactions. These differences suggest that cooperativity between FG-domains arises from FG-receptor interactions instead of FG-FG interactions. On a technical note, this work highlights an improved SMFS technique which involves pre-passivating the underlying substrate surface with polyethylene glycol to reduce undesirable AFM tip-surface effects. A high yield of acceptable data is subsequently obtained from the low surface coverage of target molecules by implementing SMFS measurements in force-volume (FV) mode.     

Dissecting the 3-D structure of vimentin intermediate filaments by cryo-electron tomography

Vimentin polymerizes via complex lateral interactions of coiled-coil dimers into long, flexible filaments referred to as intermediate filaments (IFs). Intermediate in diameter between microtubules and microfilaments, IFs constitute the third cytoskeletal filament system of metazoan cells. Here we investigated the molecular basis of the 3-D architecture of vimentin IFs by cryo-electron microscopy (cryo-EM) as well as cryo-electron tomography (Cryo-ET) 3-D reconstruction. We demonstrate that vimentin filaments in cross-section exhibit predominantly a four-stranded protofibrilar organization with a right-handed supertwist with a helical pitch of about 96 nm. Compact filaments imaged by cryo-EM appear surprisingly straight and hence appear very stiff. In addition, IFs exhibited an increased flexibility at sites of partial unraveling. This is in strong contrast to chemically fixed, negatively stained preparations of vimentin filaments that generally exhibit smooth bending without untwisting. At some point along the filament unraveling may be triggered and propagates in a cooperative manner so that long stretches of filaments appear to have unraveled rapidly in a coordinated fashion.