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 the cells of many vertebrates. They are the most abundant surface feature of many vertebrate cell types, especially endothelial cells, adipocytes and embryonic notochord cells. 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 mechanoprotection, mechanosensation, endocytosis, oncogenesis, and the uptake of pathogenic bacteria and certain viruses. Caveolin Formation and maintenance of caveolae was initially thought to be 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. Cavin proteins emerged in the late 2000s to be the main structural components controlling caveola formation. The cavin protein family consists of Cavin1 (also known as PTRF), Cavin2 (also known as SDPR), Cavin3 (also known as SRBC) and Cavin4 (also known as MURC). Cavin1 has been shown to be the main regulator of caveola formation in multiple tissues, with the sole expression of Cavin1 sufficient for morphological caveola formation in cells lacking caveolae but abundant in Cav1. Cavin4, analogous to Cav3, is muscle-specific. Caveolae are one source of clathrin-independent raft-dependent endocytosis.