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

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