Caveolae membrane

Brading How close would the caveolae membrane get to the SR where it butts through ... 

R. G. Anderson. The caveolae membrane system. Annu. Rev. Biochem. 67 199-225... 

E. J. Smart, Y. S. Ying, C. Mineo, and R. G. Anderson. A detergent-free method for purifying caveolae membrane from tissue culture cells. Proc. Natl. Acad. Sci. USA 92 10104-10108 (1995). 

K. S. Song, M. Sargiacomo, F. Galbiati, M. Parent and M. P. Lisanti, Targeting of a G alpha subunit (Gil alpha) and c-Src tyrosine kinase to caveolae membranes clarifying the role of N-myristoylation. Cell Mol Biol 43,293-303 (1997). 

K. Das, R.Y. Lewis, P.E. Scherer, and M.P. Lisanti, The membrane-spanning domains of caveolins-1 and -2 mediate the formation of caveolin hetero-oligomers. Implications for the assembly of caveolae membranes in vivo, J. Biol. Chem., 1999, 274, 18721-18728. 

Cav-1 is an integral membrane proteins and accumulates of P-catenin within caveolae membranes and thus inhibits the P-catenin/Lef-1 signaling activated by Wnt-1 or the overexpression of P-catenin itself. Recent findings indicate that Cav-1 inhibits Wnt signaling by directly interacting with P-catenin depending on its scaffolding domains (Mo et al, 2010). 

Mineo C, James GL, Smart EJ, Anderson RGW Localization of epidermal growth factor-stimulated Ras/Raf-1 interaction to caveolae membrane. J. Biol. Chem. 1996 271 11930 11935. 

Caveolae are invaginations of the plasma membrane. They contain the protein caveolin and are rich in certain phospholipids. Similar to coated pits, they bud off internally forming endocytic vesicles. Caveolae play an important role in the internalization of certain cell surface receptors. 

Fyn is a nonreceptor tyrosine kinase related to Src that is frequently found in cell junctions. Die protein is N-myristoylated and palmitoylated and thereby becomes associated with caveolae-like membrane microdomains. Fyn can interact with a variety of other signaling molecules and control a diversity of biological processes such as T cell receptor signaling, regulation of brain function, and adhesion mediated signaling. 

Lipid Rafts Caveolae Are Special Features of Some Membranes... 

While the fluid mosaic model of membrane stmcture has stood up well to detailed scrutiny, additional features of membrane structure and function are constantly emerging. Two structures of particular current interest, located in surface membranes, are tipid rafts and caveolae. The former are dynamic areas of the exo-plasmic leaflet of the lipid bilayer enriched in cholesterol and sphingolipids they are involved in signal transduction and possibly other processes. Caveolae may derive from lipid rafts. Many if not all of them contain the protein caveolin-1, which may be involved in their formation from rafts. Caveolae are observable by electron microscopy as flask-shaped indentations of the cell membrane. Proteins detected in caveolae include various components of the signal-transduction system (eg, the insutin receptor and some G proteins), the folate receptor, and endothetial nitric oxide synthase (eNOS). Caveolae and lipid rafts are active areas of research, and ideas concerning them and their possible roles in various diseases are rapidly evolving. 

New information has been added in appropriate chapters on hpid rafts and caveolae, aquaporins, connexins, disorders due to mutations in genes encoding proteins involved in intracellular membrane transport, absorption of iron, and conformational diseases and pharmacogenomics. 

Uittenbogaard, A, Everson, WV, Matveev, SV, and Smart, EJ, 2002. Cholesteryl ester is transported from caveolae to internal membranes as part of a caveolin-annexin II lipid-protein complex. J Biol Chem 277,4925—4-931. 

Although apoE HDL particles are formed by astrocytes in vitro, the brain contents of apoE knockout (-/-) were not found to differ in lipid content in comparison to those obtained from normal animals [14]. A probable explanation is that newly synthesized cholesterol can be transported from astrocyte ER to plasma membrane via an alternative route that employs caveolae to form apoAl-HDL [15]. 

Other pinocytotic pathways also exist that do not depend on either caveolae or clathrin, although these are not as well defined [55]. Specific receptors continue to be internalized in the absence of clathrin or caveolin and these pathways can be monitored by following glycosyl phos-phatidylinositol (GPI (-anchored proteins. Nonclathrin, noncaveolin pathways may also be responsible for the reuptake of membrane in neuroendocrine cells after stimulated secretion. Some, but not all, of these pathways appear to require dynamin. 

There is growing evidence that the membrane receptors for estrogens are very important in tissues as the vascular endothelium (Chambliss et al. 2002 Hodgin et al. 2002 Mendelsohn 2002 White 2002). In the endothelial cells ERs appear to be located in specific zones of the membranes called caveolas, but not in the greater part of the membrane. Such receptors mediate rapid responses to... 

FIG. 4. Ultrastructure of vascular smooth muscle of the rabbit inferior vena cava revealed with electron microscopy. Serial cross-sections of VSMCs are shown in series 1 (panel A—D) and series 2 (panel E—G). Series 1 illustrates the close spatial apposition between the superficial SR sheet and the PM with the apices of the caveolae perforating through the superficial SR sheets to come into contact with the bulk cytoplasm. The membranes of the PM (dotted line) and the SR (solid line) in panel A-D are outlined to the right of the respective panels. The close apposition between the superficial SR sheet, the PM and the neck region of the caveolae creates a narrow and expansive restricted space. Series 2 illustrates the perpendicular sheets of SR, which appear to arise from the superficial SR sheets. Mitochondria also come into close contact with the perpendicular SR sheets. Panel H contains a stylized illustration of the close association between the superficial SR sheet, which is continuous with the perpendicular sheet, the perforating caveolae (C), the PM and a mitochondrion (M). Panel I shows calyculin-A mediated dissociation of the superficial SR sheets from the PM (see arrows). The black scale bar indicated represents 200 nm of distance. 

Hlaustein The question is whether those holes are holes in the barrier or not. If you look at the reticulum where the caveolae stick up, this is not a hole in the barrier because the diffusion is restricted within the barrier itself. These holes are places where the plasma membrane at the top of the caveolae can actually communicate with bulk cytoplasm. This is a different region than that between the plasma membrane and the reticulum. 

FIG. 5. View of the SR network in a longitudinally oriented rabbit portal vein smooth muscle cell stained with osmium ferricyanide. Note the close apposition of the SR to the plasma membrane (small arrows) and caveolae, as well as its relationship to mitochondria (M) in some instances the SR completely surrounds mitochondria. 

Bolton I want to ask your opinion on a quantitative point. How far are the caveolae and their associated SR from the plasma membrane My point is, if it is a small distance, let s say some 0.2 /tm, which is just below the resolution of the confocal microscope so that the caveolae, their SR and the plasma membrane cannot be separately resolved, then the plasma membrane ought to stain strongly with DiOCg or labelled ryanodine. Is this the case ... 

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