Journal of Lipid Research Volume 51, 2010 1093anism by which dietary fat is delivered to peripheral targeted tissues, muscle, heart, and adipose tissue. The majority of the triacylglycerol (TAG) in the chylomicron is derived from hydrolytic products of dietary lipids that are resynthesized to TAG by the intestinal ER. The newly synthesized TAG crosses the ER membrane and forms the prechylomicron in the ER lumen in a two-step process ( 3 ). The exit step of the prechylomicron from the ER is the rate-limiting step by which dietary TAG traverses the intestinal absorptive cell ( 4 ) ( 5 ). Once detached from the ER membrane, the PCTV dock and then fuse with the Golgi using the N-ethylmaleimide sensitive factor attachment protein receptor (SNARE) complex composed of VAMP7 (the R-SNARE), syntaxin5 (the Qa-SNARE), vti1A (the Qb-SNARE), and Bet1 (the Qc-SNARE) ( 2 ). Our interest in the molecular mechanism of the docking of PCTV with the Golgi in relation to the COPII proteins was raised by these fi ndings: (a) Although PCTV could be generated from ER membranes in the absence of Sar1, the initiator of the COPII complex, the vesicles formed could not fuse with the Golgi ( 1 ). The vesicles were of the same size as PCTV generated using native cytosol, contained apolipoproteinB48 (apoB48), the quintessential chylomicron apolipoprotein, and VAMP7, the R-SNARE of the PCTV-Golgi SNARE complex; however, neither docking nor fusion was possible using these vesicles. b) The liver-fatty acid binding protein (L-FABP) can, in the absence of additional cytosolic components, elaborate PCTV from intestinal ER membranes ( 6 ). However, in support of the data obtained using vesicles formed in the absence of Sar1, these L-FABP formed vesicles do not contain COPII proteins and cannot fuse with the Golgi. In Abstract The rate-limiting step in the transit of dietary fat across the intestinal absorptive cell is its exit from the endoplasmic reticulum (ER) in a specialized ER-to-Golgi transport vesicle, the prechylomicron transport vesicle (PCTV). PCTV bud off from the ER membranes and have unique features; they are the largest ER-derived vesicles (average diameter 250 nm), do not require GTP and COPII proteins for their formation, and utilize VAMP7 as a v-N-ethylmaleimide sensitive factor attachment protein receptor (SNARE). However, PCTV require COPII proteins for their fusion with the Golgi, suggesting a role for them in Golgi target recognition. In support of this, PCTV contained each of the fi ve COPII proteins when docked with the Golgi. When PCTV were fused with the Golgi, the COPII proteins were present in greatly diminished amounts, indicating they had cycled back to the cytosol. Immuno-depletion of Sec31 from the cytosol did not affect PCTV-Golgi docking, but depletion of Sec23 resulted in a 25% decrease. Immunodepletion of Sec24C caused a nearly complete cessation of PCTV docking activity, but on the addition of recombinant Sec24C, docking activity was restored. We conclude that the COPII proteins are present at docking of PCTV with...