Sphingomyelin metabolism controls the shape and function of the Golgi cisternae

The flat Golgi cisterna is a highly conserved feature of eukaryotic cells, but how is this morphology achieved and is it related to its function in cargo sorting and export? A physical model of cisterna morphology led us to propose that sphingomyelin (SM) metabolism at the trans-Golgi membranes in mammalian cells essentially controls the structural features of a Golgi cisterna by regulating its association to curvature-generating proteins. An experimental test of this hypothesis revealed that affecting SM homeostasis converted flat cisternae into highly curled membranes with a concomitant dissociation of membrane curvature-generating proteins. These data lend support to our hypothesis that SM metabolism controls the structural organization of a Golgi cisterna. Together with our previously presented role of SM in controlling the location of proteins involved in glycosylation and vesicle formation, our data reveal the significance of SM metabolism in the structural organization and function of Golgi cisternae.

Golgi apparatus self-organizes into the characteristic shape via postmitotic reassembly dynamics

The Golgi apparatus is a membrane-bounded organelle with the characteristic shape of a series of stacked flat cisternae. During mitosis in mammalian cells, the Golgi apparatus is once fragmented into small vesicles and then reassembled to form the characteristic shape again in each daughter cell. The mechanism and details of the reassembly process remain elusive. Here, by the physical simulation of a coarse-grained membrane model, we reconstructed the three-dimensional morphological dynamics of the Golgi reassembly process. Considering the stability of the interphase Golgi shape, we introduce two hypothetical mechanisms—the Golgi rim stabilizer protein and curvature-dependent restriction on membrane fusion—into the general biomembrane model. We show that the characteristic Golgi shape is spontaneously organized from the assembly of vesicles by proper tuning of the two additional mechanisms, i.e., the Golgi reassembly process is modeled as self-organization. We also demonstrate that the fine Golgi shape forms via a balance of three reaction speeds: vesicle aggregation, membrane fusion, and shape relaxation. Moreover, the membrane fusion activity decreases thickness and the number of stacked cisternae of the emerging shapes.

The Biogenesis of the Golgi Ribbon: The Roles of Membrane Input from the ER and of GM130

The Golgi complex in mammalian cells forms a continuous ribbon of interconnected stacks of flat cisternae. We show here that this distinctive architecture reflects and requires the continuous input of membranes from the endoplasmic reticulum (ER), in the form of pleiomorphic ER-to-Golgi carriers (EGCs). An important step in the biogenesis of the Golgi ribbon is the complete incorporation of the EGCs into the stacks. This requires the Golgi-matrix protein GM130, which continuously cycles between the cis-Golgi compartments and the EGCs. On acquiring GM130, the EGCs undergo homotypic tethering and fusion, maturing into larger and more homogeneous membrane units that appear primed for incorporation into the Golgi stacks. In the absence of GM130, this process is impaired and the EGCs remain as distinct entities. This induces the accumulation of tubulovesicular membranes, the shortening of the cisternae, and the breakdown of the Golgi ribbon. Under these conditions, however, secretory cargo can still be delivered to the Golgi complex, although this occurs less efficiently, and apparently through transient and/or limited continuities between the EGCs and the Golgi cisternae.

Fine Structure Study of Pollen Development in Haemanthus katherinae Baker

The newly formed vegetative cell of the pollen grain of the African blood lily has a spheroidal nucleus, few dictyosomes, and a small amount of endoplasmic reticulum. Plastids are smaller than those of the microspore and usually lack starch granules. Mitochondria and lipid bodies are more numerous than they were in the microspores, but their appearance is unchanged. As the pollen grain matures, the vegetative nucleus becomes irregular in shape. There is a dramatic increase in the number of dictyosomes, starch accumulates in the plastids, and a moderately well developed system of endoplasmic reticulum appears in the form of flat cisternae. In the developmental period immediately preceding anthesis, the vegetative nucleus becomes lobate and small nucleoli replace the large nucleoli present earlier. Plastids lose their starch, lipid bodies disappear, and the endoplasmic reticulum becomes vesicular in this final stage before germination.

A PECULIAR CONFIGURATION OF AGRANULAR RETICULUM (CANALICULATE LAMELLAR BODY) IN THE RAT PINEALOCYTE

A cytoplasmic structure exhibiting a peculiar configuration of agranular reticulum has been found in the rat pinealocyte, and has been designated a canaliculate lamellar body. It consists of a number of fenestrated, flat cisternae which are closely spaced. They bear some distant resemblance to the annulate lamellae previously reported in a variety of cell types. Profiles of stacks of lamellae in a plane of section always display two distinct aspects, the surface and the cross-sectional views of flat cisternae. A surface view shows the hexagonal arrangement of pores or fenestrations. The pores in successive lamellae are aligned precisely, one behind the other, so that clear, cylindrical channels are seen running perpendicular to the lamellae as indicated by transverse sections of the lamellar stacks. Large canaliculate lamellar bodies are composed of many extended series of lamellar stacks which pursue a tortuous course and cross one another. Occasionally the canaliculate lamellar body is located deep in a nuclear invagination, which reminds one of the so-called nuclear pellets (Kernkugeln) reported by light microscopy. The functional significance of the body is unknown.