VIP21-caveolin, a membrane protein constituent of the caveolar coat, oligomerizes in vivo and in vitro.

Abstract

VIP21-caveolin is a membrane protein, proposed to be a component of the striated coat covering the cytoplasmic surface of caveolae. To investigate the biochemical composition of the caveolar coat, we used our previous observation that VIP21-caveolin is present in large complexes and insoluble in the detergents CHAPS or Triton X-114. The mild treatment of these insoluble structures with sodium dodecyl sulfate leads to the detection of high molecular mass complexes of approximately 200, 400, and 600 kDa. The 400-kDa complex purified to homogeneity from dog lung is shown to consist exclusive of the two isoforms of VIP21-caveolin. Pulse-chase experiments indicate that the oligomers form early after the protein is synthesized in the endoplasmic reticulum (ER). VIP21-caveolin does indeed insert into the ER membrane through the classical translocation machinery. Its hydrophobic domain adopts an unusual loop configuration exposing the N-and C-flanking regions to the cytoplasm. Similar high molecular mass complexes can be produced from the in vitro-synthesized VIP21-caveolin. The complex formation occurs only if VIP21-caveolin isoforms are properly inserted into the membrane; formation is cytosol-dependent and does not involve a vesicle fusion step. We propose that high molecular mass oligomers of VIP21-caveolin represent the basic units forming the caveolar coat. They are formed in the ER and later, between the ER and the plasma membrane, these oligomers could associate into larger detergent-insoluble structures.

Ancient origin of reggie (flotillin), reggie-like, and other lipid-raft proteins: convergent evolution of the SPFH domain

“…Even unrelated proteins such as caveolins perform comparable scaffolding functions in analogous caveolae DRM structures [64]. Interestingly, caveolins present certain structural similarities to reggies/flotillins, such as palmitoylation sites [65] and oligomers in DRM fractions [66]. The large sequence divergence between reggies/flotillins and reggie-like proteins is likely to be a reflection of their independent origin.…”

Src-induced Phosphorylation of Caveolin-2 on Tyrosine 19

“…At the level of the ER, caveolin-1 and -2 are known to tightly interact and they form a high molecular mass hetero-oligomer, containing ϳ14 -16 individual caveolin molecules (5,40). At the level of the Golgi, these ϳ300ϳ350-kDa hetero-oligomer units are thought to undergo a second stage of oligomerization via homotypic C-terminal interactions, giving rise to the formation of a caveolae-sized vesicle that is targeted to the plasma membrane (41).…”

And still they are moving… Dynamic properties of caveolae

“…First, the addition of cholesterol-binding drugs like filipin or nystatin changes the morphology of caveolae, making them flat [18,25]. Second, caveolae are found in a Triton X-114-insoluble floating fraction (TIFF) [23], which is enriched in glycosphingolipids as well as in cholesterol [26,27]. Moreover, as was recently shown, VIP21-caveolin itself could be a cholesterol-binding protein [28].…”

“…The protein forms an unusual hairpin loop structure in the membrane, with two cytoplasmically exposed regions flanking a very long hydrophobic segment [21][22][23]. In addition, the protein has two quite remarkable properties which are possibly connected to the structure of caveolae: it is insoluble in non-ionic detergents such as Triton X-100 or CHAPS [13,20] and it can associate with itself to form high molecular mass homo-oligomers in vivo as well as in vitro [23,24]. These oligomers could be structural elements of the caveolar coat.…”