The TRAPP complex: insights into its architecture and function.

Vesicle-mediated transport is a process carried out by virtually every cell and is required for the proper targeting and secretion of proteins. As such, there are numerous players involved to ensure that the proteins are properly localized. Overall, transport requires vesicle budding, recognition of the vesicle by the target membrane and fusion of the vesicle with the target membrane resulting in delivery of its contents. The initial interaction between the vesicle and the target membrane has been referred to as tethering. Because this is the first contact between the two membranes, tethering is critical to ensuring that specificity is achieved. It is therefore not surprising that there are numerous 'tethering factors' involved ranging from multisubunit complexes, coiled-coil proteins and Rab guanosine triphosphatases. Of the multisubunit tethering complexes, one of the best studied at the molecular level is the evolutionarily conserved TRAPP complex. There are two forms of this complex: TRAPP I and TRAPP II. In yeast, these complexes function in a number of processes including endoplasmic reticulum-to-Golgi transport (TRAPP I) and an ill-defined step at the trans Golgi (TRAPP II). Because the complex was first reported in 1998 (1), there has been a decade of studies that have clarified some aspects of its function but have also raised further questions. In this review, we will discuss recent advances in our understanding of yeast and mammalian TRAPP at the structural and functional levels and its role in disease while trying to resolve some apparent discrepancies and highlighting areas for future study.