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 five 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. Immuno-depletion 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 the Golgi and that Sec24C is required for this event. Sec23 plays a less important role.
Highlights
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)
Our interest in the molecular mechanism of the docking of PCTV with the Golgi in relation to the COPII proteins was raised by these findings: (a) PCTV could be generated from ER membranes in the absence of Sar[1], the initiator of the COPII complex, the vesicles formed could not fuse with the Golgi (1)
COPII proteins are present at PCTV docking with the Golgi In contrast to older information (18), more recent data would suggest that ER-to-Golgi transport vesicles which contain COPII proteins are attached to Golgi tethering proteins prior to their eventual fusion with the Golgi (13)
Summary
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). The prechylomicron transport vesicle (PCTV) is an enlarged, COPII-containing vesicle that transports the developing chylomicron from the ER (endoplasmic reticulum) to the Golgi (1, 2). This largest of the lipoproteins is uniquely formed in the intestine and is the primary mechanism by which dietary fat is delivered to peripheral targeted tissues, muscle, heart, and adipose tissue. Our interest in the molecular mechanism of the docking of PCTV with the Golgi in relation to the COPII proteins was raised by these findings: (a) PCTV could be generated from ER membranes in the absence of Sar[1], the initiator of the COPII complex, the vesicles formed could not fuse with the Golgi (1).
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