Large tethering complexes mediate the initial interaction of a transport vesicle with its target membrane. There are two forms of the multi-subunit tethering complex called TRAPP (TRAPPI and TRAPPII) that tether transport vesicles in different trafficking steps. Understanding TRAPP complex assembly and the protein-protein interactions among the subunits is an important step in elucidating the function of this tether. Here we have used several different approaches to study the protein-protein interactions among the subunits of the TRAPP complexes in both mammalian cells and yeast. Our studies have revealed that the low molecular weight subunits of TRAPP form two subcomplexes in vitro. One subcomplex contains mammalian BET3 (mBET3), mTRS31 and mTRS20, while mBET5 and mTRS23 form a second subcomplex. Furthermore, mBET3 directly interacts with mBET5 in vitro. Our findings also suggest that the TRAPPII-specific subunit, yTrs120p (yeast Trs120p), binds to the periphery of the TRAPPII complex. Although the non-essential TRAPP subunit yTrs33p interacts with yBet3p, yTrs33p is not required for TRAPP complex assembly. Together our findings indicate that BET3 plays an important role in the organization of the TRAPP complexes in both mammalian cells and yeast. (c) 2006 Elsevier Inc. All rights reserved.