Caveolae

In biology, caveolae (Latin for "little caves"; singular, caveola), which are a special type of lipid raft, are small (50–100 nanometer) invaginations of the plasma membrane in many vertebrate cell types, especially in endothelial cells and adipocytes. They were originally discovered by E. Yamada in 1955.

These flask-shaped structures are rich in proteins as well as lipids such as cholesterol and sphingolipids and have several functions in signal transduction. They are also believed to play a role in endocytosis, oncogenesis, and the uptake of pathogenic bacteria and certain viruses.

Formation and maintenance of caveolae is primarily due to caveolin, a 21 kD protein. There are three homologous genes of caveolin expressed in mammalian cells: Cav1, Cav2 and Cav3. These proteins have a common topology: cytoplasmic N-terminus with scaffolding domain, long hairpin transmembrane domain and cytoplasmic C-terminus. Caveolins are synthesized as monomers and transported to the Golgi apparatus. During their subsequent transport through the secretory pathway, caveolins associate with lipid rafts and form oligomers (14-16 molecules). These oligomerized caveolins form the caveolae. The presence of caveolin leads to a local change in morphology of the membrane.

Caveolae are one source of clathrin-independent raft-dependent endocytosis. The ability of caveolins to oligomerize due to their oligomerization domains is necessary for formation of caveolar endocytic vesicles. The oligomerization leads to formation of caveolin-rich microdomains in the plasma membrane. Increased levels of cholesterol and insertion of the scaffolding domains of caveolins into the plasma membrane leads to the expansion of the caveolar invagination and the formation of endocytic vesicles. Fission of the vesicle from the plasma membrane is then mediated by GTPase dynamin II, which is localized at the neck of the budding vesicle. The released caveolar vesicle can fuse with early endosome or caveosome. The caveosome is an endosomal compartment with neutral pH which does not have early endosomal markers, however, contains molecules internalized by the caveolar endocytosis.

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