Although many of the eukaryotic proteins involved in the dynamic behavior of membranes - in cell movement, endocytosis, exocytosis, and cell division - have been identified, many of the mechanisms by which they curve and pinch membranes into different configurations have yet to be elucidated. Pietro De Camilli (Yale University, New Haven, USA) showed, using fluorescence and electron microscopy combined with high-resolution structures, how the dimeric, banana-shaped BAR and F-BAR families of proteins interact with membranes to create and sense membrane curvature, which is necessary to form transport vesicles. Using synthetic giant unilamellar vesicles and fluorescence microscopy, Randy Schekman (University of California, Berkeley, USA), similarly demonstrated that the Sar1 GTPase, required for COP II vesicle trafficking of cargo between the endoplasmic reticulum and the Golgi complex, plays a direct role in creating membrane curvature for vesicle formation and scission. This reaction was highly dependent upon GTP and the amino-terminal helix of Sar1. Jenny Hinshaw (National Institute of Diabetes and Digestive and Kidney Diseases, NIH, Bethesda, USA) studies the large GTPase dynamin, and a related protein Dnm1, using cryo-electron microscopy. She presented remarkable images showing how the proteins self-associate on lipid tubules in a helical fashion. Upon GTP hydrolysis, they wrap tightly around each other to squeeze the membranes together, resulting in membrane constriction and scission.
In contrast to proteins that cause vesicle curvature and scission from the outside of the vesicle neck, the endosomal sorting complex required for transport III (ESCRT III) complex is used both for formation of internal endosome vesicles (the opposite orientation from budding of transport vesicles) and for the budding of HIV particles from the outside of the cell. Chris Hill (University of Utah, Salt Lake City, USA) described how the ESCRT III subunits assemble into spirals that pinch off newly forming vesicles from the inside. Currently, the field awaits even higher-resolution information, especially that of specific protein-lipid and protein-protein interactions, in order to fully understand these mechanisms of vesicle scission.
The nuclear pore complex (NPC) was seemingly unrelated to vesicle budding, but turns out to have some striking similarities. Stephen Brohawn (Massachusetts Institute of Technology, Cambridge, USA) presented high-resolution structural data showing that several of the NPC subunits are homologous to the coat proteins (such as COP II) that shape newly forming vesicles, indicating a common ancestor for proteins that bend membranes. Andrej Sali (University of California, San Francisco, USA) presented a tour de force of computational, electron microscopic, biochemical and mass spectrometric approaches that has succeeded in modeling the entire NPC, which contains multiple copies of up to 30 different proteins. Several of these proteins line the pore and are responsible for bending the membranes to form the pore, while others on the inside of the complex regulate traffic in and out of the pore.