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. 2008 Nov 25;6(11):e292. doi: 10.1371/journal.pbio.0060292

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This is an open-access article distributed under the terms of the Creative Commons Public Domain declaration which stipulates that, once placed in the public domain, this work may be freely reproduced, distributed, transmitted, modified, built upon, or otherwise used by anyone for any lawful purpose.

This is an open-access article distributed under the terms of the Creative Commons Public Domain declaration, which stipulates that, once placed in the public domain, this work may be freely reproduced, distributed, transmitted, modified, built upon, or otherwise used by anyone for any lawful purpose.

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COPI and COPII trafficking systems shuttle proteins and lipids between ER and Golgi. Small TIP47-coated lipid droplets are born at the ER and reside in the cytosol. The bigger lipid droplets are coated by ADRP, and TIP47 is returned to the ER. ATGL can translocate between the cytosol and the lipid droplet surface, where it begins the conversion of TG to DG and eventually NEFA. Other TG lipases may also be present (indicated by the second arrow liberating TG from the hydrophobic lipid droplet core). Wild-type Golgi–ER transport function of COPI might be necessary for proper lipid droplet biogenesis at the ER. COPI could directly function at the lipid droplet surface, mediating a similar ER-targeted transport function as reported for the Golgi–ER COPI transport. COPI is a positive regulator of ATGL with a unresolved mechanism (indicated by the question mark) and a negative regulator of TIP47 lipid droplet localization. ADRP and TIP47 are negative regulators of ATGL activity at the lipid droplet surface.