Coatomer vesicles are not required for inhibition of Golgi transport by G-protein activators

Membrane Curvature, Trans-Membrane Area Asymmetry, Budding, Fission and Organelle Geometry

In biology, the modern scientific fashion is to mostly study proteins. Much less attention is paid to lipids. However, lipids themselves are extremely important for the formation and functioning of cellular membrane organelles. Here, the role of the geometry of the lipid bilayer in regulation of organelle shape is analyzed. It is proposed that during rapid shape transition, the number of lipid heads and their size (i.e., due to the change in lipid head charge) inside lipid leaflets modulates the geometrical properties of organelles, in particular their membrane curvature. Insertion of proteins into a lipid bilayer and the shape of protein trans-membrane domains also affect the trans-membrane asymmetry between surface areas of luminal and cytosol leaflets of the membrane. In the cases where lipid molecules with a specific shape are not predominant, the shape of lipids (cylindrical, conical, or wedge-like) is less important for the regulation of membrane curvature, due to the flexibility of their acyl chains and their high ability to diffuse.

Trans-membrane area asymmetry controls the shape of cellular organelles

Membrane organelles often have complicated shapes and differ in their volume, surface area and membrane curvature. The ratio between the surface area of the cytosolic and luminal leaflets (trans-membrane area asymmetry (TAA)) determines the membrane curvature within different sites of the organelle. Thus, the shape of the organelle could be critically dependent on TAA. Here, using mathematical modeling and stereological measurements of TAA during fast transformation of organelle shapes, we present evidence that suggests that when organelle volume and surface area are constant, TAA can regulate transformation of the shape of the Golgi apparatus, endosomal multivesicular bodies, and microvilli of brush borders of kidney epithelial cells. Extraction of membrane curvature by small spheres, such as COPI-dependent vesicles within the Golgi (extraction of positive curvature), or by intraluminal vesicles within endosomes (extraction of negative curvature) controls the shape of these organelles. For instance, Golgi tubulation is critically dependent on the fusion of COPI vesicles with Golgi cisternae, and vice versa, for the extraction of membrane curvature into 50–60 nm vesicles, to induce transformation of Golgi tubules into cisternae. Also, formation of intraluminal ultra-small vesicles after fusion of endosomes allows equilibration of their TAA, volume and surface area. Finally, when microvilli of the brush border are broken into vesicles and microvilli fragments, TAA of these membranes remains the same as TAA of the microvilli. Thus, TAA has a significant role in transformation of organelle shape when other factors remain constant.

Delivery of the malaria virulence protein PfEMP1 to the erythrocyte surface requires cholesterol-rich domains

The particular virulence of the human malaria parasite Plasmodium falciparum derives from export of parasite-encoded proteins to the surface of the mature erythrocytes in which it resides. The mechanisms and machinery for the export of proteins to the erythrocyte membrane are largely unknown. In other eukaryotic cells, cholesterol-rich membrane microdomains or "rafts" have been shown to play an important role in the export of proteins to the cell surface. Our data suggest that depletion of cholesterol from the erythrocyte membrane with methyl-beta-cyclodextrin significantly inhibits the delivery of the major virulence factor P. falciparum erythrocyte membrane protein 1 (PfEMP1). The trafficking defect appears to lie at the level of transfer of PfEMP1 from parasite-derived membranous structures within the infected erythrocyte cytoplasm, known as the Maurer's clefts, to the erythrocyte membrane. Thus our data suggest that delivery of this key cytoadherence-mediating protein to the host erythrocyte membrane involves insertion of PfEMP1 at cholesterol-rich microdomains. GTP-dependent vesicle budding and fusion events are also involved in many trafficking processes. To determine whether GTP-dependent events are involved in PfEMP1 trafficking, we have incorporated non-membrane-permeating GTP analogs inside resealed erythrocytes. Although these nonhydrolyzable GTP analogs reduced erythrocyte invasion efficiency and partially retarded growth of the intracellular parasite, they appeared to have little direct effect on PfEMP1 trafficking.

In vitro transport on cis and trans sides of the Golgi involves two distinct types of coatomer and ADP-ribosylation factor-independent transport intermediates

The cisternal maturation model proposes that secretory proteins transit the Golgi in cisternae that mature by the continuous retrograde transport of Golgi enzymes in vesicles. We have tested the hypothesis that de novo generation of transport intermediates containing medial, trans, and trans Golgi network (TGN) enzymes is reconstituted in vitro. Our analysis shows that the majority of transport is mediated by a steady state of transport intermediate production and consumption by Golgi cisternae, with only a minor contribution of pre-existing transport intermediates. Transport in the medial and trans regions of the stack involved intermediates containing Golgi enzymes, apparently moving in a retrograde direction. In contrast, transport between the trans Golgi and TGN was exclusively mediated by intermediates containing secretory protein, as expected for anterograde transport. These intermediates may be physiologically relevant, because only these two specific types of intermediates can be detected in cell homogenates. By analogy to the coatomer (COPI)-independent transport of Golgi enzymes to the endoplasmic reticulum, the steady-state production of intra-Golgi transport intermediates was not impaired by inhibition of COPI vesicle formation. These data suggest a model for COPI-independent intra-Golgi transport by cisternal maturation with a shift in mechanism to anterograde transport at the trans Golgi and TGN boundary.

Dynamics of the COPII coat with GTP and stable analogues

We have developed an assay to monitor the assembly of the COPII coat onto liposomes in real time. We show that with Sar1pGTP bound to liposomes, a single round of assembly and disassembly of the COPII coat lasts a few seconds. The two large COPII complexes Sec23/24p and Sec13/31p bind almost instantaneously (in less than 1 s) to Sar1pGTP-doped liposomes. This binding is followed by a fast (less than 10 s) disassembly due to a 10-fold acceleration of the GTPase-activating protein activity of Sec23/24p by the Sec13/31p complex. Experiments with the phosphate analogue BeFx suggest that Sec23/24p provides residues directly involved in GTP hydrolysis on Sar1p.

Gut thoughts on the Golgi complex

The new millennium coincides within 1 year of Camillo Golgi's centennial celebrations. It is quite remarkable that the structure and formation of this organelle is as controversial today as was its mere existence from Golgi's time to the 1950s, when EM approaches were introduced. Since the late 1950s, two opposing models of Golgi structure and function have split the Golgi scientific community, namely vesicular transport versus organelle maturation. Although a few years ago Golgi maturation seemed to be 'out for the count', it has recently seen an almost messianic revival. In this review, I argue that this large-scale desertion from the vesicle transport model to the maturation camp is premature. I propose an alternative, dynamic steady-state model, in which transient tubular connections function in parallel to vesicular transport and that the biosynthetic pathway is made up of three major distinct compartments: the ER, the Golgi and the TGN.

Regulators and effectors of the ARF GTPases

The small G proteins of the ARF family are key regulators of membrane dynamics. Many functions of ARF proteins in cells are being revealed by studies of their regulators and effectors. Significant progress has been made over the past year, with the identification of a surprisingly large family of novel ARF GTPase-activating proteins. In addition, two new classes of effectors, the PIP kinases and a novel family of monomeric coat-like proteins have been discovered.