The talks covered transport (and interactions) between many different membrane compartments. Ludger Johannes, the session chair, discussed the mechanism of clathrin-independent endocytosis, describing how the plasma membrane can bud inwards in the absence of coat proteins like clathrin or caveolin. Jacques Neefjes, on the other hand, discussed how vesicles containing MHC Class II molecules are transported to the surface of antigen-presenting cells.
Diana Zala (from Frederic Saudou's lab) described her recent paper about the rapid transport of vesicles along motor axons. The vesicles are moved by microtubule-based motor proteins and, given the enormous length of motor axons, this requires an awful lot of energy. Zala and colleagues found that this energy is provided by the glycolysis reaction, which can take place on the surface of the vesicles themselves due to the recruitment of glycolytic enzymes like GAPDH. In the absence of vesicular GAPDH, the vesicles are transported along axons much more slowly, indicating that rapid and efficient axonal transport requires the vesicles to be “energy-independent”.
Antonella De Matteis discussed the non-vesicular transport of the lipid glucosylceramide, a key precursor of glycosphingolipids. Glucosylceramide is transported through the Golgi by the lipid transfer protein FAPP2, which extracts glucosylceramide from the cis-Golgi membrane and deposits it at the trans-Golgi network (TGN). De Matteis showed how binding to glucosylceramide increases FAPP2’s affinity for the phospholipid PI(4)P, which helps target the protein to the TGN. Intriguingly, glucosylceramide can also be transported through the Golgi stack via a vesicle-based mechanism. De Matteis showed FAPP2 is converted into different types of glycosphingolipid, depending on whether it passes through each Golgi cisterna in turn, or whether it is transported directly to the TGN by FAPP2.
And finally, Francesca Giordano (from Pietro De Camilli's laboratory) described how a family of proteins called extended synaptotagmins helps connect the endoplasmic reticulum to the plasma membrane. These proteins localize to contact sites and contacts are lost when the extended synaptotagmins are depleted from human cells. However, calcium homeostasis (which is thought to be the main function of ER-plasma membrane contacts) isn’t affected in the absence of the extended synaptotagmins, so the physiological function of these proteins remains unclear.