Component of the TRAPP I, TRAPP II and TRAPP III complexes which act as guanine nucleotide exchange factors (GEF) for YPT1. TRAPP I plays a key role in the late stages of endoplasmic reticulum to Golgi traffic. TRAPP II plays a role in intra-Golgi transport. TRAPP III plays a role in autophagosome formation. Part of the multisubunit TRAPP (transport protein particle) I complex composed of BET3, BET5, TRS20, TRS23, TRS31 and TRS33. Part of the multisubunit TRAPP (transport protein particle) II complex composed of BET3, BET5, TRS20, TRS23, TRS31, TRS33, TRS65, TRS85, TRS120 and TRS130. Part of the multisubunit TRAPP (transport protein particle) III complex composed of BET3, BET5, TRS20, TRS23, TRS31, TRS33 and TRS85. Present with 8350 molecules/cell in log phase SD medium. Belongs to the TRAPP small subunits family. BET3 subfamily.