Available online at www.sciencedirect.com ScienceDirect Morphology and function of membrane-bound organelles 1 2 Rebecca Heald and Orna Cohen-Fix The cell interior is a busy and crowded place. A large fraction of membranes are lipid bilayers made predominantly of the cell volume is taken up by organelles that come in a variety phospholipids and proteins, both of which can contribute of shapes and sizes. These organelles are surrounded by to membrane curvature. A difference in lipid composition membrane that not only acts as a diffusion barrier, but also between the two bilayers can itself lead to membrane provides each organelle with its unique morphology that curvature, and this likely drives the formation of the rims contributes to its function, often in ways that are poorly of Golgi cisternae and the tubular structures that connect understood. Here we discuss recent discoveries on the the Golgi stack to form the ribbon [8]. Recently, a novel ER relationship between organelle structure and function. structure made of a helicoidal surface was shown to connect Addresses adjacent ER sheets [9 ] (Figure 1b). This configuration, 1 Department of Molecular and Cell Biology, University of California, which is akin to the ramps of a parking garage, appears to be Berkeley, CA 94720, United States an energetically favorable structure that allows the dense 2 The Laboratory of Cell and Molecular Biology, NIDDK, NIH, Bethesda, packing of ER sheets to accommodate maximum protein MD 20892, United States synthesis in secretory cells. Thus, inherent properties of Corresponding authors: Heald, Rebecca ([email protected]) and membranes contribute to their degree of curvature. Cohen-Fix, Orna ([email protected]) Proteins also contribute to membrane curvature, as in the case of the ER [10] (Figure 1c). Proteins called reticulons Current Opinion in Cell Biology 2014, 26:79–86 and DP1/Yop1/REEPs contain hydrophobic domains that This review comes from a themed issue on Cell architecture form wedges on one side of the lipid bilayer, forcing it to Edited by Sue Biggins and Matthew D Welch bend towards the opposite side. These proteins are For a complete overview see the Issue and the Editorial essential for maintaining ER tubules and are also involved in the highly curved regions of the NE where Available online 16th November 2013 nuclear pore complexes are embedded. The tubular ER 0955-0674/$ – see front matter, # 2013 Elsevier Ltd. All rights reserved. network is also shaped by the formation of three-way http://dx.doi.org/10.1016/j.ceb.2013.10.006 junctions, generated by homotypic membrane fusion between the tip of one ER tubule and the side of another in a process mediated by a conserved family of proteins Introduction called atlastin/Sey1 [11 ,12 ]. Other proteins that contrib- Organelles are dynamic, changing size and shape to ute to membrane curvature are the BAR domain proteins, maintain homeostasis and adjusting to the various needs which form a rigid crescent-shaped structure and force of the cell. Some changes occur as part of the normal cell membrane bending through electrostatic interactions be- cycle, for example during cell division [1–3]. Other tween the concave surface of the protein dimer and the changes happen in response to challenges or stress and membrane [13]. Proteins also contribute to the constant reflect a modification in organelle function, such as a luminal width of low curvature double-membrane struc- change in protein folding capacity of the endoplasmic tures, such as ER cisternae and the nuclear envelope, by reticulum (ER) or ATP production in mitochondria [4,5]. acting as spacers within the luminal space [14 ,15]. Thus, It is assumed that alterations to organelle morphology organelle morphology is driven, in part, by dedicated reflect an underlying functional optimization. Yet, this proteins that affect membrane curvature and geometry. relationship is often poorly understood: for example, does the peripheral endoplasmic reticulum (ER) have to be in Proteins that are not dedicated to altering membrane the shape of tubules in order to carry out its function? shape may also contribute to organelle structure. For Does mitochondrial size matter? In this review we discuss example, the curvature of the cristae of the mitochondrial recent advances in our understanding of the relationship inner membrane is stabilized by the presence of ATP between organelle structure and function, focusing prim- synthase [16,17], and the ER sheets are likely stabilized arily on the ER, nucleus and mitochondria. The reader is by attached ribosomes [18]. Finally, membrane shape can referred to excellent reviews that cover earlier work on be affected by external cytoskeletal forces. One such Golgi [1], peroxosime [6] and endosome [7] structure. example is the formation of ER tubules through the attachment of the ER to microtubule associated proteins Shaping a membrane-bound organelle and the pulling forces exerted by microtubule elongation How are organelles shaped? The morphology of most and microtubule motors [19–21]. The combination of organelles is characterized by a combination of flat and lipid and protein composition, along with external forces, curved membrane, such as in the ER (Figure 1a). Cellular provides each organelle with its unique morphology. www.sciencedirect.com Current Opinion in Cell Biology 2014, 26:79–86 80 Cell architecture Figure 1 (a) (c) a b c d (b) e s Pa GT Current Opinion in Cell Biology Diverse membrane structures in the ER. (a) The ER is an interconnected network composed of branched tubules and sheets, some of which can form stacks, as shown in the illustration. ER tubules are stabilized by the oligomerization of proteins such as reticulons and DP1/Yop1/REEps (in red), while 3-way junctions are mediated by proteins such as atlastins (in blue). (b) The structure of reticulons and atlastin. Membrane curvature is induced by the insertion of protein ‘wedges’ (two in the case of reticulons and one in the case of atlastin) that traverse only part way through the lipid bilayer, forcing the membrane to curve. Atlastin has in addition GTPase activity that is necessary for fusing membranes and generating 3-way junctions. (c) Helicoidal membrane structure in stacked ER sheets. A 3D reconstruction of a region of stacked ER sheets from an acinar cell of a mouse salivary gland. The letters (a–d) mark the ER sheets that are connected through a helicoidal structure (to the left). From Ref. [9 ]. Complex shapes allow for distinct functions were shown to affect mitochondrial fission [24 ]: ER within a single organelle tubules wrap around mitochondria at future fission sites While some organelles, such as the nucleus or the and can constrict mitochondria even in the absence of the vacuole, are simple in shape, other organelles, such as mitochondrial fission machinery, such as the dynamin the Golgi and the ER, have complex shapes made up of a related protein Dnm1/Drp1. The mechanism by which network of cisternae and tubules. This complexity allows this constriction occurs, and whether ER tubules affect for segregation of biochemical processes within the orga- the structure of other organelles, remain to be deter- nelle: for example, ribosomes are preferentially associated mined. with the flat ER membrane that forms the rough ER [14 ,18]. In contrast, ER tubules are engaged in processes Unlike the ER, the metazoan nucleus is usually a round such as lipid synthesis and they are responsible for the or oval structure with limited membrane curvature, majority of contacts between the ER and other organelles. except where the inner and outer nuclear membranes Indeed, when the tubular structure of the ER is disrupted meet at nuclear pores. Thus, distinct domains within the in yeast, the efficiency of lipid transfer between the ER nucleus, if they exist, are not defined by nuclear mem- and mitochondria is reduced [22 ]. The ER forms mem- brane-derived compartments. The budding yeast brane contact sites (MCS) with virtually all organelles in nucleus, however, changes shape during the cell cycle: the cell; the juxtaposed membranes are typically 20 nm while the metazoan nucleus disassembles in mitosis, the apart and they are held together by protein complexes budding yeast nucleus remains intact, and in anaphase it that are unique to each organelle [23]. Traditionally, MCS forms an hourglass shape with only a narrow opening were thought of as sites for inter-organelle communi- connecting the nuclear compartments within the mother 2+ cation, such as exchange of Ca and the transfer of lipids, and daughter cells. This opening is sufficiently small to which are synthesized predominantly in the ER but are form a diffusion barrier between the two nuclear halves, needed by all membrane-bound organelles. More allowing the asymmetric accumulation of a transcription recently, MCS between the ER and the mitochondria factor only in the daughter nuclear compartment [25 ]. Current Opinion in Cell Biology 2014, 26:79–86 www.sciencedirect.com Organelle structure and function Heald and Cohen-Fix 81 Widening of the opening connecting the mother and remodel to avoid obstructing the mitotic apparatus under daughter nuclear halves by genetic manipulations non-pathological conditions? allowed diffusion of proteins between the two compart- ments, indicating that budding yeast cells take advantage Following NE break down in vertebrate cells, many NE of cell cycle changes in nuclear shape to compartmenta- components are resorbed into the ER. The ER itself is lize the nucleus. conspicuously absent from the area of the mitotic spindle and becomes enriched at spindle poles [18]. Two recent Organelle shape and cell function papers describe microtubule-dependent mechanisms that It is likely that organelle shape, and in particular the serve to keep the ER clear of the mitotic spindle, and in membrane configuration, has evolved to suit not only the one case this is essential for proper NE architecture organelle’s biological activity, but also overall cell func- (Figure 2).