PAT family proteins pervade lipid droplet cores.
The PAT family proteins, named after perilipin, adipophilin, and the tail-interacting protein of 47 kDa (TIP47), are implicated in intracellular lipid metabolism. They associate with lipid droplets, but how is completely unclear. From immunofluorescence studies, they are reported to be restricted to the outer membrane monolayer enveloping the lipid droplet and not to enter the core. Recently, we found another kind of lipid droplet-associated protein, caveolin-1, inside lipid droplets. Using freeze-fracture immunocytochemistry and electron microscopy, we now describe the distributions of perilipin and caveolin-1 and of adipophilin and TIP47 in lipid droplets of adipocytes and macrophages. All of these lipid droplet-associated proteins pervade the lipid droplet core and hence are not restricted to the droplet surface. Moreover, lipid droplets are surprisingly heterogeneous with respect to their complements and their distribution of lipid droplet-associated proteins. Whereas caveolin-1 is synthesized in the endoplasmic reticulum and is transferred to the lipid droplet core by inundating lipids during droplet budding, the PAT proteins, which are synthesized on free ribosomes in the cytoplasm, evidently target to the lipid droplet after it has formed. How the polar lipid droplet-associated proteins are accommodated among the essentially hydrophobic neutral lipids of the lipid droplet core remains to be determined.
Robenek
Horst
H
Institute for Arteriosclerosis Research, University of Münster, Münster, Germany. robenek@uni-muenster.de
Robenek
Mirko J
MJ
Troyer
David
D
D017667:Adipocytes / Q000378:metabolism; D002352:Carrier Proteins; D051242:Caveolin 1; D022461:Caveolins / Q000378:metabolism; D002460:Cell Line; D003593:Cytoplasm / Q000378:metabolism; D004268:DNA-Binding Proteins / Q000502:physiology*; D004721:Endoplasmic Reticulum / Q000378:metabolism; D005614:Freeze Fracturing; D006801:Humans; D007150:Immunohistochemistry; D047908:Intracellular Signaling Peptides and Proteins / Q000502:physiology*; D050356:Lipid Metabolism; D008264:Macrophages / Q000378:metabolism; D008565:Membrane Proteins; D008854:Microscopy, Electron; D008856:Microscopy, Fluorescence; D010455:Peptides / Q000378:metabolism / Q000502:physiology*; D000070778:Perilipin-1; D000070780:Perilipin-2; D000070796:Perilipin-3; D010750:Phosphoproteins / Q000378:metabolism / Q000502:physiology*; D011257:Pregnancy Proteins / Q000502:physiology*; D012270:Ribosomes / Q000378:metabolism; D033921:Vesicular Transport Proteins
C495540:CAV1 protein, human; D002352:Carrier Proteins; D051242:Caveolin 1; D022461:Caveolins; D004268:DNA-Binding Proteins; D047908:Intracellular Signaling Peptides and Proteins; D008565:Membrane Proteins; C000602982:PLIN2 protein, human; C038196:PLIN3 protein, human; D010455:Peptides; D000070778:Perilipin-1; D000070780:Perilipin-2; D000070796:Perilipin-3; D010750:Phosphoproteins; D011257:Pregnancy Proteins; D033921:Vesicular Transport Proteins