The TRAPP complexes: oligomeric exchange factors that activate the small GTPases Rab1 and Rab11

The Transport Protein Particle (TRAPP) complexes are highly conserved multisubunit complexes that act as nucleotide exchange factors (GEFs) for Rab GTPases. They act in both protein secretion and autophagy and have also been proposed to have a role in other processes such as cytokinesis and ciliogenesis. There are two TRAPP complexes in metazoans: TRAPPII, which activates Rab11; and TRAPPIII, which activates Rab1. Both complexes share a core of small subunits that form the active site for the exchange of GDP for GTP. In addition, each TRAPP complex has distinct large subunits that determine the specificity of each complex towards its substrate Rab and are essential for activity in vivo. Crystal structures have revealed the organisation of the TRAPP core and the mechanism of Rab1 activation, whilst recent cryo-EM structures have unveiled the arrangement of the specific subunits around the core to form each complex. Combining these findings with functional experiments has allowed the proposal of mechanisms for how the specificity of each complex towards their cognate Rab is determined and for the arrangement of these large complexes on the membrane.