The TRAPP complex

Abstract

Intracellular transport of biosynthetic cargo from the endoplasmic reticulum to the plasma membrane occurs by membrane-bound vesicular or tubulove-sicular carriers that dissociate from a donor compartment and fuse with an acceptor compartment. The arrival of a transport carrier to the correct destination along the exocytic pathway is important for the appropriate spatio-temporal processing and delivery of the cargo molecules. Membrane fusion requires the formation of a trans SNARE complex (SNAREpin) of SNARE proteins contributed by the donor and acceptor membranes that is thought to overcome the energy barrier that would prevent two membranes from fusing. Prior to this event, however, proteins called tethering factors appear to act as physical links between membrane compartments (Whyte and Munro 2002). In addition to acting as physical links the tethering factors may contribute to the specificity of compartmental fusion by their interaction with various molecules on the donor and acceptor membranes. Two broad classes of tethering factors are represented by proteins containing extensive coiled-coil domains (such as p115/Uso1 in mammals/yeast) or by a number of large multiprotein complexes that mediate membrane traffic between various compartments within the cell (Whyte and Munro 2002). A common feature of the tethering factors seems to be their interaction with small GTPases of the Rab/Ypt family and with SNAREs that appears to contribute to the specificity of membrane fusion.