Passive transport of Ca ions through lipid bilayers imaged by widefield second harmonic microscopy.

In biology, release of Ca ions in the cytosol is essential to trigger or control many cell functions. Calcium signaling acutely depends on lipid membrane permeability to Ca. For proper understanding of membrane permeability to Ca, both membrane hydration and the structure of the hydrophobic core must be taken into account.

Mechanochemical Rules for Shape-Shifting Filaments that Remodel Membranes.

The sequential exchange of filament composition to increase filament curvature was proposed as a mechanism for how some biological polymers deform and cut membranes. The relationship between the filament composition and its mechanical effect is lacking. We develop a kinetic model for the assembly of composite filaments that includes protein-membrane adhesion, filament mechanics and membrane mechanics.

Snf7 spirals sense and alter membrane curvature.

Endosomal Sorting Complex Required for Transport III (ESCRT-III) is a conserved protein system involved in many cellular processes resulting in membrane deformation and scission, topologically away from the cytoplasm. However, little is known about the transition of the planar membrane-associated protein assembly into a 3D structure. High-speed atomic force microscopy (HS-AFM) provided insights into assembly, structural dynamics and turnover of Snf7, the major ESCRT-III component, on planar supported lipid bilayers.

Principles of membrane remodeling by dynamic ESCRT-III polymers.

Endosomal protein complex required for transport-III (ESCRT-III) polymers are involved in many crucial cellular functions, from cell division to endosome-lysosome dynamics. As a eukaryotic membrane remodeling machinery, ESCRT-III is unique in its ability to catalyze fission of membrane necks from their luminal side and to participate in membrane remodeling processes of essentially all cellular organelles. Found in Archaea, it is also the most evolutionary ancient membrane remodeling machinery.

Anisotropic ESCRT-III architecture governs helical membrane tube formation.

ESCRT-III proteins assemble into ubiquitous membrane-remodeling polymers during many cellular processes. Here we describe the structure of helical membrane tubes that are scaffolded by bundled ESCRT-III filaments. Cryo-ET reveals how the shape of the helical membrane tube arises from the assembly of two distinct bundles of helical filaments that have the same helical path but bind the membrane with different interfaces.

A fluorescent membrane tension probe.

Cells and organelles are delimited by lipid bilayers in which high deformability is essential to many cell processes, including motility, endocytosis and cell division. Membrane tension is therefore a major regulator of the cell processes that remodel membranes, albeit one that is very hard to measure in vivo. Here we show that a planarizable push-pull fluorescent probe called FliptR (fluorescent lipid tension reporter) can monitor changes in membrane tension by changing its fluorescence lifetime as a function of the twist between its fluorescent groups.

Systematic analysis of complex genetic interactions.

To systematically explore complex genetic interactions, we constructed ~200,000 yeast triple mutants and scored negative trigenic interactions. We selected double-mutant query genes across a broad spectrum of biological processes, spanning a range of quantitative features of the global digenic interaction network and tested for a genetic interaction with a third mutation. Trigenic interactions often occurred among functionally related genes, and essential genes were hubs on the trigenic network.

Uncoupling of dynamin polymerization and GTPase activity revealed by the conformation-specific nanobody dynab.

Dynamin is a large GTPase that forms a helical collar at the neck of endocytic pits, and catalyzes membrane fission (Schmid and Frolov, 2011; Ferguson and De Camilli, 2012). Dynamin fission reaction is strictly dependent on GTP hydrolysis, but how fission is mediated is still debated (Antonny et al., 2016): GTP energy could be spent in membrane constriction required for fission, or in disassembly of the dynamin polymer to trigger fission.

Structural inhibition of dynamin-mediated membrane fission by endophilin.

Dynamin, which mediates membrane fission during endocytosis, binds endophilin and other members of the in-mphiphysin-vs (BAR) protein family. How endophilin influences endocytic membrane fission is still unclear. Here, we show that dynamin-mediated membrane fission is potently inhibited in vitro when an excess of endophilin co-assembles with dynamin around membrane tubules.

The advantage of channeling nucleotides for very processive functions.

Nucleoside triphosphate (NTP)s, like ATP (adenosine 5'-triphosphate) and GTP (guanosine 5'-triphosphate), have long been considered sufficiently concentrated and diffusible to fuel all cellular ATPases (adenosine triphosphatases) and GTPases (guanosine triphosphatases) in an energetically healthy cell without becoming limiting for function. However, increasing evidence for the importance of local ATP and GTP pools, synthesised in close proximity to ATP- or GTP-consuming reactions, has fundamentally challenged our view of energy metabolism. It has become evident that cellular energy metabolism occurs in many specialised 'microcompartments', where energy in the form of NTPs is transferred preferentially from NTP-generating modules directly to NTP-consuming modules.

Dynamic and elastic shape transitions in curved ESCRT-III filaments.

The ESCRT-III complex is an evolutionary ancient and conserved complex that catalyzes fission of lipid membranes from the lumen of the neck in many, if not all processes requiring this specific fission reaction. The ESCRT-III membrane remodeling complex is unique as its molecular and polymeric structures do not intuitively suggests how it could deform and break lipid membranes. Here we review the common structural features of the ESCRT-III subunits, and the shape diversity of the various filamentous forms.

Tensing Up for Lipid Droplet Formation.

Lipid droplets are fat storage organelles in cells that physically resemble stable oil-water emulsion droplets. In this issue of Developmental Cell, Ben M'barek et al. (2017) show that the resemblance is more than superficial: physical principles governing emulsion stability also control lipid droplet nucleation and growth.

Dynamic subunit turnover in ESCRT-III assemblies is regulated by Vps4 to mediate membrane remodelling during cytokinesis.

The endosomal sorting complex required for transport (ESCRT)-III mediates membrane fission in fundamental cellular processes, including cytokinesis. ESCRT-III is thought to form persistent filaments that over time increase their curvature to constrict membranes. Unexpectedly, we found that ESCRT-III at the midbody of human cells rapidly turns over subunits with cytoplasmic pools while gradually forming larger assemblies.

Mitochondrial Homeostasis: How Do Dimers of Mitofusins Mediate Mitochondrial Fusion?

Mitochondria have high fusion and fission rates to maintain their size and number throughout the cell cycle. How is fusion mediated? New structural studies propose mechanisms by which the dynamin-like mitofusin proteins promote fusion of mitochondria.

Dynamic remodeling of the dynamin helix during membrane constriction.

Dynamin is a dimeric GTPase that assembles into a helix around the neck of endocytic buds. Upon GTP hydrolysis, dynamin breaks these necks, a reaction called membrane fission. Fission requires dynamin to first constrict the membrane.

Structural insights into the centronuclear myopathy-associated functions of BIN1 and dynamin 2.

Centronuclear myopathies (CNMs) are genetic diseases whose symptoms are muscle weakness and atrophy (wasting) and centralised nuclei. Recent human genetic studies have isolated several groups of mutations. Among them, many are found in two interacting proteins essential to clathrin-mediated endocytosis, dynamin and the BIN-Amphiphysin-Rvs (BAR) protein BIN1/amphiphysin 2.

A 3D printed microfluidic device for production of functionalized hydrogel microcapsules for culture and differentiation of human Neuronal Stem Cells (hNSC).

We present here a microfluidic device that generates sub-millimetric hollow hydrogel spheres, encapsulating cells and coated internally with a layer of reconstituted extracellular matrix (ECM) of a few microns thick. The spherical capsules, composed of alginate hydrogel, originate from the spontaneous instability of a multi-layered jet formed by co-extrusion using a coaxial flow device. We provide a simple design to manufacture this device using a DLP (digital light processing) 3D printer.

TORC1 and TORC2 work together to regulate ribosomal protein S6 phosphorylation in Saccharomyces cerevisiae.

Nutrient-sensitive phosphorylation of the S6 protein of the 40S subunit of the eukaryote ribosome is highly conserved. However, despite four decades of research, the functional consequences of this modification remain unknown. Revisiting this enigma in Saccharomyces cerevisiae, we found that the regulation of Rps6 phosphorylation on Ser-232 and Ser-233 is mediated by both TOR complex 1 (TORC1) and TORC2.

Relaxation of Loaded ESCRT-III Spiral Springs Drives Membrane Deformation.

ESCRT-III is required for lipid membrane remodeling in many cellular processes, from abscission to viral budding and multi-vesicular body biogenesis. However, how ESCRT-III polymerization generates membrane curvature remains debated. Here, we show that Snf7, the main component of ESCRT-III, polymerizes into spirals at the surface of lipid bilayers.

When cell biology meets theory.

Cell biologists now have tools and knowledge to generate useful quantitative data. But how can we make sense of these data, and are we measuring the correct parameters? Moreover, how can we test hypotheses quantitatively? To answer these questions, the theory of physics is required and is essential to the future of quantitative cell biology.

A balance between membrane elasticity and polymerization energy sets the shape of spherical clathrin coats.

In endocytosis, scaffolding is one of the mechanisms to create membrane curvature by moulding the membrane into the spherical shape of the clathrin cage. However, the impact of membrane elastic parameters on the assembly and shape of clathrin lattices has never been experimentally evaluated. Here, we show that membrane tension opposes clathrin polymerization.

Reaching a consensus on the mechanism of dynamin?

AMONG THE PROTEINS INVOLVED IN LIPID MEMBRANE REMODELING IN INTRACELLULAR TRAFFIC, DYNAMIN HAS BEEN THE FOCUS OF MANY STUDIES, AS IT WAS THE FIRST PROTEIN SHOWN TO BE MECHANISTICALLY INVOLVED IN MEMBRANE FISSION: the reaction by which a vesicle neck can be severed to release a free vesicle. After almost 25 years of research, a wide variety of data from various techniques has been acquired on the mechanism by which dynamin breaks membranes. However, the literature may sometimes sound confusing, and the primary goal of this review will be to provide a stepping stone towards a potential consensus on how dynamin may work.

BAR domain scaffolds in dynamin-mediated membrane fission.

Biological membranes undergo constant remodeling by membrane fission and fusion to change their shape and to exchange material between subcellular compartments. During clathrin-mediated endocytosis, the dynamic assembly and disassembly of protein scaffolds comprising members of the bin-amphiphysin-rvs (BAR) domain protein superfamily constrain the membrane into distinct shapes as the pathway progresses toward fission by the GTPase dynamin. In this Review, we discuss how BAR domain protein assembly and disassembly are controlled in space and time and which structural and biochemical features allow the tight regulation of their shape and function to enable dynamin-mediated membrane fission.