TRAPP I implicated in the specificity of tethering in ER-to-Golgi transport

TRAPP is a conserved protein complex required early in the secretory pathwa y. Here, we report two forms of TRAPP, TRAPP I and TRAPP II, that mediate d ifferent transport events. Using chemically pure TRAPP I and COPII vesicles , we have reconstituted vesicle targeting in vitro. The binding of COPII ve sicles to TRAPP I is specific, blocked by GTP gammaS, and, surprisingly, do es not require other tethering factors. Our findings imply that TRAPP I is the receptor on the Golgi for COPII vesicles. Once the vesicle binds to TRA PP I, the small GTP binding protein Ypt1p is activated and other tethering factors are recruited.