THE MEMBRANE-TRANSPORT FACTOR TAP P115 CYCLES BETWEEN THE GOLGI AND EARLIER SECRETORY COMPARTMENTS AND CONTAINS DISTINCT DOMAINS REQUIRED FOR ITS LOCALIZATION AND FUNCTION/

The mammalian protein TAP/p115 and its yeast homologue Uso1p have an e ssential role in membrane traffic (Nakajima et al., 1991; Waters et al ., 1992; Sztul et al., 1993; Rabouille et al., 1995). To inquire into the site and mechanism of TAP/p115 action, we aimed to localize it and to identify domains required for its function. We show that in interp hase cells, TAP/p115 localizes predominantly to the Golgi and to perip heral structures that represent vesicular tubular clusters (VTCs) invo lved in ER to Golgi transport. Using BFA/ nocodazole treatments we con firm that TAP/p115 is present on ER to Golgi transport intermediates. TAP/ p115 redistributes to peripheral structures containing ERGIC-53 d uring a 15 degrees C treatment, suggesting that it is a cycling protei n. Within the Golgi, TAP/p115 is associated with pleiomorphic structur es on the cis side of the cis-Giolgi cisterna and the cis-most cistern a, but is not detected in more distal compartments of the Golgi. TAP/p 115 binds the cis-Golgi protein GM130, and the COOH-terminal acidic do main of TAP/p115 is required for this interaction. TAP/p115 interactio n with GM130 occurs only in the Golgi and is not required for TAP/p115 association with peripheral VTCs. To exam ine whether interaction wit h GM130 is required to recruit TAP/p115 to the Golgi, TAP/p115 mutants lacking the acidic domain were expressed and localized in transfected cells. Mutants lacking the GM130-binding domain showed normal Golgi l ocalization, indicating that TAP/p115 is recruited to the Golgi indepe ndently of its ability to bind GM130. Such mutants were also able to a ssociate with peripheral VTCs. Interestingly, TAP/p115 mutants contain ing the GM130-binding domain but lacking portions of the NH2-terminal region were restricted from the Golgi and localized to the ER. The COO H-terminal domain required for GM130 binding and the NH2-terminal regi on required for Golgi localization appear functionally relevant since expression of TAP/p115 mutants lacking either of these domains leads t o loss of normal Golgi morphology.