Caveolae at a glance.

The plasma membrane is more than a simple delimitation of the boundary of the cell but is a dynamic multi-domain membrane system participating in numerous cellular processes. In many different cell types, the plasma membrane is heavily decorated with small pits of 60–80 nm in diameter, which constitute a specialized type of microdomain called caveolae. Nowadays, almost 60 years after the first observation of caveolae (Palade, 1953), these enigmatic structures are still a hot topic in cellular biology with questions remaining regarding their formation, regulation, and functions in different cell types and in pathological scenarios. The discovery of the caveolin proteins greatly advanced the study of caveolae, particularly with the subsequent demonstration that caveolin expression induces, and is required for, the formation of caveolae (Drab et al., 2001; Fra et al., 1995; Razani et al., 2001). Caveolins are now known to constitute a family of integral membrane proteins that form the principal membrane components of caveolae. The first reported caveolin protein (also independently described as VIP-21) is now called caveolin-1 (Cav1), after the discovery of the related proteins caveolin-2 (Cav2) and caveolin-3 (Cav3). Cav1, together with Cav2, is expressed in a wide range of tissues with the highest levels of expression in endothelial cells, adipocytes, fibroblasts and smooth-muscle cells. Cav3 appears to be the sole isoform expressed in skeletal and cardiac muscle. The essential role of caveolins for caveola formation was made evident with the generation of Cav1and Cav3knockout mice, which completely lack caveolae in the respective tissues and, consequently, exhibit a range of phenotypes including muscle, pulmonary and lipid disorders (Le Lay and Kurzchalia, 2005). The recent characterization of cavin-1 (also known as polymerase I and transcript release factor, PTRF) – a protein initially described as a transcription factor and associated with caveolae in adipocytes – as an essential factor required for the stabilization of caveolae at the plasma membrane (Hill et al., 2008; Liu and Pilch, 2008) brought new insights into caveola biogenesis and suggested that caveola formation can be a dynamically regulated process. Moreover, further investigation of cavin-1 and the other three members of the cavin family in the past two years (Bastiani et al., 2009; Hansen