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6Caveolae were originally identified as flask-shaped invaginations of the plasma membrane in endothelial and epithelial cells (14). Prior to the development of biochemical methods for their purification, caveolae were thought to principally mediate the transcellular movement of molecules (101,145). Recently, the development of novel purification procedures has greatly expanded our knowledge regarding the putative functions of caveolae in vivo. In this review, we seek to update the working definition of caveolae, describe the functional roles of the caveolin gene family, and summarize the evidence that supports a role for caveolae as mediators of a number of cellular signaling processes. OVERVIEW: CAVEOLAE AND CAVEOLA-RELATED DOMAINS ARE LIQUID-ORDERED MICRODOMAINSAlthough caveolae were classically defined as plasma membrane invaginations with a characteristic diameter of ϳ50 to 100 nm, this morphological description is inadequate. Caveolae can be invaginated, flat within the plane of the plasma membrane, or detached vesicles. In addition, caveolae can fuse to form grape-like structures (132) and tubules (116) with sizes significantly larger than 100 nm. Morphologically, they are abundant in endothelia, muscle cell types, adipocytes, and lung epithelial cells (34,112). Recent investigations have also revealed that caveola-like structures are present within the nervous system (15,50,71).Caveolae have a unique lipid composition. They are mainly composed of cholesterol and sphingolipids. In contrast, noncaveolar regions of the plasma membrane are composed mainly of phospholipids. Cholesterol and sphingolipids can form a liquid-ordered (l o ) phase, which is resistant to detergent solubilization (13). These detergent-resistant liquid-ordered domains purified from mammalian cells and tissues are currently referred to as detergent-insoluble glycolipid-rich membranes, cholesterolsphingolipid rafts, glycolipid-enriched membranes, detergentresistant membranes, caveolin-enriched membranes, low-density Triton-insoluble domains, caveola-like domains, and caveolarelated domains. Here, we will refer to liquid-ordered domains that contain caveolins as caveolae and liquid-ordered domains lacking caveolins as caveola-related domains (Fig. 1). In addition, experiments with liposomes in vitro have provided evidence that cholesterol and sphingolipids alone can form liquid-ordered lipid domains which are resistant to detergent solubilization (13). The idea that caveolae and caveola-related domains are liquid-ordered membranous structures is not new and has been proposed by other investigators as well (1,12,13,120,142). For a more complete definition of liquid-ordered domains, see the work of Brown and London (12, 13). Furthermore, by using multiple independent approaches, several laboratories have now provided evidence that these microdomains exist in living cells in vivo (12,13,46,65,72,120).Caveolins are the defining protein components of caveolae. Interestingly, caveolins bind cholesterol directly. In addition, cholesterol binding...

The plasma membrane of many motile cells undergoes highly regulated protrusions and invaginations that support the formation of podosomes, invadopodia and circular dorsal ruffles. Although they are similar in appearance and in their formation--which is mediated by a highly conserved actin-membrane apparatus--these transient surface membrane distortions are distinct. Their function is to help the cell as it migrates, attaches and invades.

The yeast protein Fis1p has been shown to participate in mitochondrial fission mediated by the dynaminrelated protein Dnm1p. In mammalian cells, the dynamin-like protein DLP1/Drp1 functions as a mitochondrial fission protein, but the mechanisms by which DLP1/Drp1 and the mitochondrial membrane interact during the fission process are undefined. In this study, we have tested the role of a mammalian homologue of Fis1p, hFis1, and provided new and mechanistic information about the control of mitochondrial fission in mammalian cells. Through differential tagging and deletion experiments, we demonstrate that the intact C-terminal structure of hFis1 is essential for mitochondrial localization, whereas the N-terminal region of hFis1 is necessary for mitochondrial fission. Remarkably, an increased level of cellular hFis1 strongly promotes mitochondrial fission, resulting in an accumulation of fragmented mitochondria. Conversely, cell microinjection of hFis1 antibodies or treatment with hFis1 antisense oligonucleotides induces an elongated and collapsed mitochondrial morphology. Further, fluorescence resonance energy transfer and coimmunoprecipitation studies demonstrate that hFis1 interacts with DLP1. These results suggest that hFis1 participates in mitochondrial fission through an interaction that recruits DLP1 from the cytosol. We propose that hFis1 is a limiting factor in mitochondrial fission and that the number of hFis1 molecules on the mitochondrial surface determines fission frequency.

The dynamins comprise an expanding family of ubiquitously expressed 100-kD GTPases that have been implicated in severing clathrin-coated pits during receptor-mediated endocytosis. Currently, it is unclear whether the different dynamin isoforms perform redundant functions or participate in distinct endocytic processes. To define the function of dynamin II in mammalian epithelial cells, we have generated and characterized peptide-specific antibodies to domains that either are unique to this isoform or conserved within the dynamin family. When microinjected into cultured hepatocytes these affinity-purified antibodies inhibited clathrin-mediated endocytosis and induced the formation of long plasmalemmal invaginations with attached clathrin-coated pits. In addition, clusters of distinct, nonclathrin-coated, flask-shaped invaginations resembling caveolae accumulated at the plasma membrane of antibody-injected cells. In support of this, caveola-mediated endocytosis of labeled cholera toxin B was inhibited in antibody-injected hepatocytes. Using immunoisolation techniques an anti-dynamin antibody isolated caveolar membranes directly from a hepatocyte postnuclear membrane fraction. Finally, double label immunofluorescence microscopy revealed a striking colocalization between dynamin and the caveolar coat protein caveolin. Thus, functional in vivo studies as well as ultrastructural and biochemical analyses indicate that dynamin mediates both clathrin-dependent endocytosis and the internalization of caveolae in mammalian cells.