Le double-jeu de la protéine TIP47

[The lipid droplet: a new organelle?]

Lipid droplets (LD) are depots of neutral lipids that exist virtually in all cells. Until recently, they were considered to be in the same category as glycogen granules, simple inert storage sites for energy. There is now increasing evidence that LD interact dynamically with different organelles, probably as means of providing these organelles with lipids for their membrane expansion. However, most of the mechanisms driving LD biogenesis, growth and intracellular movement remain unknown. Recent data suggest that LD remain functionally connected to the endoplasmic reticulum (ER) membrane and represent specialized ER domains rather than independent organelles. Nevertheless, they represent important cellular structures for which dysfunctions may lead to human diseases such as lypodystrophies or neurodegenerative diseases.

Solution structure studies of monomeric human TIP47/perilipin-3 reveal a highly extended conformation

Tail-interacting protein of 47 kDa (TIP47) has two putative functions: lipid biogenesis and mannose 6-phosphate receptor recycling. Progress in understanding the molecular details of these two functions has been hampered by the lack of structural data on TIP47, with a crystal structure of the C-terminal domain of the mouse homolog constituting the only structural data in the literature so far. Our studies have first provided a strategy to obtain pure monodisperse preparations of the full-length TIP47/perilipin-3 protein, as well as a series of N-terminal truncation mutants with no exogenous sequences. These constructs have then enabled us to obtain the first structural characterization of the full-length protein in solution. Our work demonstrates that the N-terminal region of TIP47/perilipin-3, in contrast to the largely helical C-terminal region, is predominantly β-structure with turns and bends. Moreover, we show that full-length TIP47/perilipin-3 adopts an extended conformation in solution, with considerable spatial separation of the N- and C-termini that would likely translate into a separation of functional domains.

TIP47 protects mitochondrial membrane integrity and inhibits oxidative-stress-induced cell death

We found that overexpression of tail interacting protein of 47 kDa (TIP47), but not its truncated form (t-TIP47) protected NIH3T3 cells from hydrogen-peroxide-induced cell death, prevented the hydrogen-peroxide-induced mitochondrial depolarization determined by 5,50,6,60-tetrachloro-1,10,3,30-tetraethyl-benzimidazolylcarbocyanine iodide (JC1), while suppression of TIP47 in HeLa cells facilitated oxidative-stress-induced cell death. TIP47 was located to the cytoplasm of untreated cells, but some was associated to mitochondria in oxidative stress. Recombinant TIP47, but not t-TIP47 increased the mitochondrial membrane potential (Deltapsi), and partially prevented Ca2+ induced depolarization. It is assumed that TIP47 can bind to mitochondria in oxidative stress, and inhibit mitochondria mediated cell death by protecting mitochondrial membrane integrity.

Localisation of caveolin in mammary tissue depends on cell type

Caveolins, components of caveolae, are expressed in mammary tissue. In order to determine whether caveolins are present in different mammary cell types and whether their localisation depends on the physiological stage or species, cav-1 and cav-2 were characterised by immunoblotting in mammary tissues from the mouse, ewe and rabbit and localised, by immunofluorescence and electron microscopy, in mammary tissues from the mouse and ewe. At all the physiological stages studied, cav-1 and cav-2 were present in endothelial and myoepithelial cells in which flask-shaped caveolae were abundant. However, labelling of cav-1 and cav-2 associated with small vesiculo-tubular structures (including those close to lipid droplets) was low in epithelial cells. To study the possible association of cav-1 with lipid droplets, lactating ewe mammary fragments were treated in vitro with brefeldin A. This treatment did not modify the association of cav-1-labelled structures with lipid droplets. Finally, HC11 and MCF-10A mammary cell lines were treated with oleic acid. The total quantity of cav-1 was little affected by the treatment, although the lipid droplet labelling of cav-1 was amplified in MCF-10A cells. Thus, the synthesis and localisation of caveolins are mostly dependent upon the cell types of mammary tissue and upon their state of differentiation.