...one GEF complex, TRAPP, might sequentially regulate two very different Rab GTPases.

Central to controlling the flow of membrane traffic through cells are Rab GTPases, which oscillate between active GTP-bound and inactive GDP-bound conformations. Their active state is regulated, in part, by guanine-exchange factors (GEFs) that catalyse the exchange of GDP for GTP. In Nature Cell Biology, Morozova et al. now report how one GEF complex, TRAPP, might sequentially regulate two very different Rab GTPases.

TRAPP is a multisubunit complex that regulates the yeast Rab protein Ypt1 — which regulates the entry of cargo vesicles into the Golgi apparatus — as well as Ypt31 and Ypt32, which both control the subsequent exit of the cargo from the Golgi. TRAPP comes in two forms, TRAPPI and TRAPPII, which have different cellular localizations. Both share a core of seven subunits, although the larger TRAPPII complex contains an additional three subunits.

Morozova et al. showed that the mutation of the TRAPPII-specific subunit Trs130 abolished the capability of isolated TRAPP complexes to activate Ypt31. By contrast, mutant TRAPP complexes showed a significantly increased activation of Ypt1. By isolating the two TRAPP complexes from wild-type yeast on the basis of their different sizes, TRAPPI and TRAPPII were shown to have opposite specificities for the Ypt GTPases.

The authors showed a physical interaction between Trs130 and Ypt31, which is physiologically important: the overexpression of Trs130 rescued the growth of yeast cells that expressed mutant Ypt31. This effect was specific for Ypt31 mutants over Ypt1 mutants, which confirmed that TRAPPII functions as a GEF for Ypt31, whereas TRAPPI activates Ypt1.

Morozova et al. proposed that rather than two discrete TRAPP complexes being expressed at different points in the Golgi, TRAPPI complexes mature into TRAPPII complexes. After the Ypt1- and TRAPPI-dependent entry of cargo into the Golgi, a subcomplex that includes Trs130 associates with TRAPPI, thereby converting it to a TRAPPII complex. TRAPPII, through activating Ypt31, then polices the exit of cargo from the Golgi. This proposal is supported by the fact that the Golgi itself was recently shown to form by the gradual maturation of cisternae, rather than having stable compartments between which cargoes are shuttled.