Lipid droplets in adipocytes serve as the principal longterm energy storage depot of animals. There is increasing recognition that lipid droplets are not merely a static neutral lipid storage site, but in fact dynamic and multifunctional organelles. Structurally, lipid droplet consists of a neutral lipid core surrounded by a phospholipid monolayer and proteins embedded in or bound to the phospholipid layer. Proteins on the surface of lipid droplets are crucial to droplet structure and dynamics. To understand the lipid droplet-associated proteome of primary adipocyte with a large central lipid droplet, lipid droplets of white adipose tissue from C57BL/6 mice were isolated. And the proteins were extracted and analyzed by liquid chromatography coupled with tandem mass spectrometry. A total of 193 proteins including 73 previously unreported proteins were identified. Furthermore, the isotope-coded affinity tags (ICAT) was used to compare the difference of lipid droplet-associated proteomes between the normal lean and the high-fat diet-induced obese C57BL/6 mice. Of 23 proteins quantified by ICAT analysis, 3 proteins were upregulated and 4 proteins were down-regulated in the lipid droplets of adipose tissue from the obese mice. Importantly, two structural proteins of lipid droplets, perilipin A and vimentin, were greatly reduced in the lipid droplets of the adipose tissue from the obese mice, implicating reduced protein machinery for lipid droplet stability.

The regulation of protein tyrosine phosphorylation is an important aspect during the cell cycle. From G2-M transition to mitotic anaphase, phosphorylation of Tyr421, Tyr466 and Tyr482 of cortactin, an actin-filament associated protein, is dramatically induced. The phosphorylated cortactin is almost exclusively associated with centrosomes or spindle poles during mitosis. At G2-M transition prior to the breakdown of the nuclear envelope, two duplicated centrosomes migrate towards opposite ends of the nucleus to form the spindle poles. This centrosome-separation process and also the start of mitosis are inhibited or delayed by the depolymerization of actin filaments. Also inhibited is the separation of centrosomes when a truncated form of cortactin is expressed, whose C-terminus contains the tyrosine phosphorylation region but lacks the actin-binding domains. We introduced mutations at the tyrosine phosphorylation sites in the truncated C-terminus of cortactin and found that the C-terminus could no longer interfere with centrosome separation process. Our study shows that, cortactin phosphorylated at Tyr421, Tyr466 and Tyr482 mediates the actin-filament-driven centrosome separation at G2-M transition by providing a bridge between the centrosome and actin-filaments.

Heat shock protein 90 (Hsp90) is an abundant molecular chaperone with two isoforms, Hsp90α and Hsp90β. Hsp90β deficiency causes embryonic lethality, whereas Hsp90α deficiency causes few abnormities except male sterility. In this paper, we reported that Hsp90α was exclusively expressed in the retina, testis, and brain. Its deficiency caused retinitis pigmentosa (RP), a disease leading to blindness. In Hsp90α-deficient mice, the retina was deteriorated and the outer segment of photoreceptor was deformed. Immunofluorescence staining and electron microscopic analysis revealed disintegrated Golgi and aberrant intersegmental vesicle transportation in Hsp90α-deficient photoreceptors. Proteomic analysis identified microtubule-associated protein 1B (MAP1B) as an Hsp90α-associated protein in photoreceptors. Hspα deficiency increased degradation of MAP1B by inducing its ubiquitination, causing α-tubulin deacetylation and microtubule destabilization. Furthermore, the treatment of wild-type mice with 17-DMAG, an Hsp90 inhibitor of geldanamycin derivative, induced the same retinal degeneration as Hsp90α deficiency. Taken together, the microtubule destabilization could be the underlying reason for Hsp90α deficiency-induced RP.