Oligomerization but Not Membrane Bending Underlies the Function of Certain F-BAR Proteins in Cell Motility and Cytokinesis

Nine unanswered questions about cytokinesis

Experiments on model systems have revealed that cytokinesis in cells with contractile rings (amoebas, fungi, and animals) depends on shared molecular mechanisms in spite of some differences that emerged during a billion years of divergent evolution. Understanding these fundamental mechanisms depends on identifying the participating proteins and characterizing the mechanisms that position the furrow, assemble the contractile ring, anchor the ring to the plasma membrane, trigger ring constriction, produce force to form a furrow, disassemble the ring, expand the plasma membrane in the furrow, and separate the daughter cell membranes. This review reveals that fascinating questions remain about each step.

Nanoscale architecture of the Schizosaccharomyces pombe contractile ring

The contractile ring is a complex molecular apparatus which physically divides many eukaryotic cells. Despite knowledge of its protein composition, the molecular architecture of the ring is not known. Here we have applied super-resolution microscopy and FRET to determine the nanoscale spatial organization of Schizosaccharomyces pombe contractile ring components relative to the plasma membrane. Similar to other membrane-tethered actin structures, we find proteins localize in specific layers relative to the membrane. The most membrane-proximal layer (0–80 nm) is composed of membrane-binding scaffolds, formin, and the tail of the essential myosin-II. An intermediate layer (80–160 nm) consists of a network of cytokinesis accessory proteins as well as multiple signaling components which influence cell division. Farthest from the membrane (160–350 nm) we find F-actin, the motor domains of myosins, and a major F-actin crosslinker. Circumferentially within the ring, multiple proteins proximal to the membrane form clusters of different sizes, while components farther from the membrane are uniformly distributed. This comprehensive organizational map provides a framework for understanding contractile ring function.

Phosphoinositide-mediated ring anchoring resists perpendicular forces to promote medial cytokinesis

Altering phosphoinositide composition through deletion of efr3, a PI4 kinase scaffold, results in type V myosin-dependent cytokinetic ring sliding in Schizosaccharomyces pombe. Membrane-binding proteins contribute to ring anchoring to resist perpendicular forces and thereby promote medial cytokinesis.

Many eukaryotic cells divide by assembling and constricting an actin- and myosin-based contractile ring (CR) that is physically linked to the plasma membrane (PM). In this study, we report that Schizosaccharomyces pombe cells lacking efr3, which encodes a conserved PM scaffold for the phosphatidylinositol-4 kinase Stt4, build CRs that can slide away from the cell middle during anaphase in a myosin V–dependent manner. The Efr3-dependent CR-anchoring mechanism is distinct from previously reported pathways dependent on the Fes/CIP4 homology Bin-Amphiphysin-Rvs167 (F-BAR) protein Cdc15 and paxillin Pxl1. In efr3Δ, the concentrations of several membrane-binding proteins were reduced in the CR and/or on the PM. Our results suggest that proper PM lipid composition is important to stabilize the central position of the CR and resist myosin V–based forces to promote the fidelity of cell division.

The Role of BAR Domain Proteins in the Regulation of Membrane Dynamics

Many cellular processes are associated with membrane remodeling. The BAR domain protein family plays a key role in the formation and detection of local membrane curvatures and in attracting other proteins, including the regulators of actin dynamics. Based on their structural and phylogenetic properties, BAR domains are divided into several groups which affect membrane in various ways and perform different functions in cells. However, recent studies have uncovered evidence of functional differences even within the same group. This review discusses the principles underlying the interactions of different groups of BAR domains, and their individual representatives ,with membranes.

Structural organization of membrane-inserted hexamers formed by Helicobacter pylori VacA toxin

Helicobacter pylori colonizes the human stomach and is a potential cause of peptic ulceration or gastric adenocarcinoma. H. pylori secretes a pore-forming toxin known as vacuolating cytotoxin A (VacA). The 88 kDa secreted VacA protein, composed of an N-terminal p33 domain and a C-terminal p55 domain, assembles into water-soluble oligomers. The structural organization of membrane-bound VacA has not been characterized in any detail and the role(s) of specific VacA domains in membrane binding and insertion are unclear. We show that membrane-bound VacA organizes into hexameric oligomers. Comparison of the two-dimensional averages of membrane-bound and soluble VacA hexamers generated using single particle electron microscopy reveals a structural difference in the central region of the oligomers (corresponding to the p33 domain), suggesting that membrane association triggers a structural change in the p33 domain. Analyses of the isolated p55 domain and VacA variants demonstrate that while the p55 domain can bind membranes, the p33 domain is required for membrane insertion. Surprisingly, neither VacA oligomerization nor the presence of putative transmembrane GXXXG repeats in the p33 domain is required for membrane insertion. These findings provide new insights into the process by which VacA binds and inserts into the lipid bilayer to form membrane channels.

How curvature-generating proteins build scaffolds on membrane nanotubes

Bin/Amphiphysin/Rvs (BAR) domain proteins control the curvature of lipid membranes in endocytosis, trafficking, cell motility, the formation of complex subcellular structures, and many other cellular phenomena. They form 3D assemblies that act as molecular scaffolds to reshape the membrane and alter its mechanical properties. It is unknown, however, how a protein scaffold forms and how BAR domains interact in these assemblies at protein densities relevant for a cell. In this work, we use various experimental, theoretical, and simulation approaches to explore how BAR proteins organize to form a scaffold on a membrane nanotube. By combining quantitative microscopy with analytical modeling, we demonstrate that a highly curving BAR protein endophilin nucleates its scaffolds at the ends of a membrane tube, contrary to a weaker curving protein centaurin, which binds evenly along the tube's length. Our work implies that the nature of local protein-membrane interactions can affect the specific localization of proteins on membrane-remodeling sites. Furthermore, we show that amphipathic helices are dispensable in forming protein scaffolds. Finally, we explore a possible molecular structure of a BAR-domain scaffold using coarse-grained molecular dynamics simulations. Together with fluorescence microscopy, the simulations show that proteins need only to cover 30-40% of a tube's surface to form a rigid assembly. Our work provides mechanical and structural insights into the way BAR proteins may sculpt the membrane as a high-order cooperative assembly in important biological processes.

Multiscale simulations of protein-facilitated membrane remodeling

Protein-facilitated shape and topology changes of cell membranes are crucial for many biological processes, such as cell division, protein trafficking, and cell signaling. However, the inherently multiscale nature of membrane remodeling presents a considerable challenge for understanding the mechanisms and physics that drive this process. To address this problem, a multiscale approach that makes use of a diverse set of computational and experimental techniques is required. The atomistic simulations provide high-resolution information on protein-membrane interactions. Experimental techniques, like electron microscopy, on the other hand, resolve high-order organization of proteins on the membrane. Coarse-grained (CG) and mesoscale computational techniques provide the intermediate link between the two scales and can give new insights into the underlying mechanisms. In this Review, we present the recent advances in multiscale computational approaches established in our group. We discuss various CG and mesoscale approaches in studying the protein-mediated large-scale membrane remodeling.

Overview of fission yeast septation

Cytokinesis is the final process of the vegetative cycle, which divides a cell into two independent daughter cells once mitosis is completed. In fungi, as in animal cells, cytokinesis requires the formation of a cleavage furrow originated by constriction of an actomyosin ring which is connected to the plasma membrane and causes its invagination. Additionally, because fungal cells have a polysaccharide cell wall outside the plasma membrane, cytokinesis requires the formation of a septum coincident with the membrane ingression. Fission yeast Schizosaccharomyces pombe is a unicellular, rod-shaped fungus that has become a popular model organism for the study of actomyosin ring formation and constriction during cell division. Here we review the current knowledge of the septation and separation processes in this fungus, as well as recent advances in understanding the functional interaction between the transmembrane enzymes that build the septum and the actomyosin ring proteins.

Linking up at the BAR: Oligomerization and F-BAR protein function

As cells grow, move, and divide, they must reorganize and rearrange their membranes and cytoskeleton. The F-BAR protein family links cellular membranes with actin cytoskeletal rearrangements in processes including endocytosis, cytokinesis, and cell motility. Here we review emerging information on mechanisms of F-BAR domain oligomerization and membrane binding, and how these activities are coordinated with additional domains to accomplish scaffolding and signaling functions.

F-BAR/EFC Domain Proteins: Some Assembly Required

Polymeric spirals of crescent-shaped BAR-domain superfamily proteins are touted to girdle eukaryotic phospholipid bilayers into narrow tubules for trafficking and membrane remodeling events. But McDonald et al. (2015) in this issue of Developmental Cell question whether this broadly held view and conceptually appealing mechanism for membrane sculpting is really overhyped.

The Tubulation Activity of a Fission Yeast F-BAR Protein Is Dispensable for Its Function in Cytokinesis

F-BAR proteins link cellular membranes to the actin cytoskeleton in many biological processes. Here we investigated the function of the Schizosaccharomyces pombe Imp2 F-BAR domain in cytokinesis and find that it is critical for Imp2's role in contractile ring constriction and disassembly. To understand mechanistically how the F-BAR domain functions, we determined its structure, elucidated how it interacts with membranes, and identified an interaction between dimers that allows helical oligomerization and membrane tubulation. Using mutations that block either membrane binding or tubulation, we find that membrane binding is required for Imp2's cytokinetic function but that oligomerization and tubulation, activities often deemed central to F-BAR protein function, are dispensable. Accordingly, F-BARs that do not have the capacity to tubulate membranes functionally substitute for the Imp2 F-BAR, establishing that its major role is as a cell-cycle-regulated bridge between the membrane and Imp2 protein partners, rather than as a driver of membrane curvature.