The differential protein and lipid compositions of noncaveolar lipid microdomains and caveolae

Morphologically, caveolae and lipid rafts are two different membrane structures. They are often reported to share similar lipid and protein compositions, and are considered to be two subtypes of membrane lipid microdomains. By modifying sucrose density gradient flotation centrifugation, which is used to isolate lipid microdomains, we were able to separate caveolae and noncaveolar lipid microdomains into two distinct fractions. The caveolar membranes are membrane vesicles of 100-nm diameter, enriched with caveolin-1 and flotillin-1. The noncaveolar lipid microdomains are amorphous membranes and most likely the coalescence of heterogeneous lipid rafts. They are depleted of caveo- lin-1 and are more enriched with cholesterol and sphingolipids than the caveolae. Many membrane proteins, such as insulin-like growth factor-1 receptor （membrane receptor）, aquaporin-1 （membrane transporter）, Thy-1 and N- cadherin （glycosylphosphatidylinositol-anchored membrane protein and membrane glycoprotein）, are specifically as- sociated with noncaveolar lipid microdomains, but not with caveolae. These results indicate that the lipid and protein compositions of caveolae differ from those of noncaveolar lipid microdomains. The difference in their protein compo- sitions implies that these two membrane microdomains may have different cellular functions.

The molecular chaperone Hsp90α deficiency causes retinal degeneration by disrupting Golgi organization and vesicle transportation in photoreceptors

Heat shock protein 90(Hsp90)is an abundant molecular chaperone with two isoforms,Hsp90α and Hsp90p.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 IB(MAP1B)as an Hsp90α-associated protein in photoreceptors.Hspcx deficiency increased degradation of MAP1B by inducing its ubiquitination,causing a-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.

The molecular chaperone Hsp90 maintains Golgi organization and vesicular trafficking by regulating microtubule stability

Hsp90 is an abundant and special molecular chaperone considered to be the regulator of many transcription factors and signaling kinases. Its high abundance is indicative of its involvement in some more fundamental processes. In this study, we provide evidence that Hsp90 is required for microtubule stabilization, Golgi organization, and vesicular trafficking. We showed that Hsp90 is bound to microtubule-associated protein 4 (MAP4), which is essential for maintaining microtubule acetylation and stabilization. Hsp90 depletion led to the decrease in MAP4, causing microtubule deacetylation and destabilization. Furthermore, in Hsp90-depleted cells, the Golgi apparatus was fragmented and anterograde vesicle trafficking was impaired, with phenotypes similar to those induced by silencing MAP4. These disruptive effects of Hsp90 depletion could be rescued by the expression of exogenous MAP4 or the treatment of trichostatin A that increases microtubule acetylation as well as stability. Thus, microtubule stability is an essential cellular event regulated by Hsp90.