Plays a role in the TGF-beta/activin signaling pathway. It associates with inactive heteromeric TGF-beta and activin receptor complexes, mainly through the type II receptor, and is released upon activation of signaling. May recruit SMAD4 to the vicinity of the receptor complex and facilitate its interaction with receptor-regulated Smads, such as SMAD2. {ECO:0000269|PubMed:11278302, ECO:0000269|PubMed:9545258}. Plays a role in vesicle-mediated protein trafficking of the endocytic membrane transport pathway. Believed to act as a component of the putative CORVET endosomal tethering complexes which is proposed to be involved in the Rab5-to-Rab7 endosome conversion probably implicating MON1A/B, and via binding SNAREs and SNARE complexes to mediate tethering and docking events during SNARE-mediated membrane fusion. The CORVET complex is proposed to function as a Rab5 effector to mediate early endosome fusion probably in specific endosome subpopulations (PubMed:25266290). Functions predominantly in APPL1- containing endosomes and in degradative but not recycling trafficking of endocytosed cargo (PubMed:25266290). {ECO:0000269|PubMed:25266290, ECO:0000305|PubMed:25266290}. This is the function of TGFBRAP1 (transforming growth factor beta receptor associated protein 1, ENSG00000135966).