TRANSCYTOSIS-ASSOCIATED PROTEIN (TAP)/P115 IS A GENERAL FUSION FACTOR REQUIRED FOR BINDING OF VESICLES TO ACCEPTOR MEMBRANES

Transcytosis-associated protein (TAP) is found on transcytotic vesicles (TCVs) and is required for their fusion with the target membrane, We developed a cell-free assay capable of differentiating targeting/binding of TCVs to membrane from later fusion events, We found that TAP mediates stable association of TCVs with the target membrane, The sequence of rat liver TAP (959-amino acid open reading frame) encodes a protein that contains (i) an N-terminal region (amino acids 1-649), (ii) an internal region with several coiled-coil stretches (amino acids 650-930), and (iii) a C-terminal acidic region (amino acids 931-959), Comparisons between TAP and other sequences indicate that TAP is identical to p115, a protein involved in cis to medial Golgi transport, and homologous to Uso1p, a yeast protein involved in endoplasmic reticulum to Golgi transport. Our findings suggest that TAP/p115/Usop1 is a general factor acting within the secretory and endocytic pathways to bind transport vesicles prior to membrane fusion.