The cell biology of caveolae is a rapidly growing area of biomedical research. Caveolae are known primarily for their ability to transport molecules across endothelial cells, but modern cellular techniques have dramatically extended our view of caveolae. They form a unique endocytic and exocytic compartment at the surface of most cells and are capable of importing molecules and delivering them to specific locations within the cell, exporting molecules to extracellular space, and compartmentalizing a variety of signaling activities. They are not simply an endocytic device with a peculiar membrane shape but constitute an entire membrane system with multiple functions essential for the cell. Specific diseases attack this system: Pathogens have been identified that use it as a means of gaining entrance to the cell. Trying to understand the full range of functions of caveolae challenges our basic instincts about the cell.

The Caveolae Membrane System

Simian virus 40 (SV40) is unusual among animal viruses in that it enters cells through caveolae, and the internalized virus accumulates in a smooth endoplasmic reticulum (ER) compartment Using video-enhanced, dual-colour, live fluorescence microscopy, we show the uptake of individual virus particles in CV-1 cells After associating with caveolae, SV40 leaves the plasma membrane in small, caveolin-1-containing vesicles It then enters larger, peripheral organelles with a non-acidic pH Although rich in caveolin-1, these organelles do not contain markers for endosomes, lysosomes, ER or Golgi, nor do they acquire ligands of clathrin-coated vesicle endocytosis After several hours in these organelles, SV40 is sorted into tubular, caveolin-free membrane vesicles that move rapidly along microtubules, and is deposited in perinuclear, syntaxin 17-positive, smooth ER organelles The microtubule-disrupting agent nocodazole inhibits formation and transport of these tubular carriers, and blocks viral infection Our results demonstrate the existence of a two-step transport pathway from plasma-membrane caveolae, through an intermediate organelle (termed the caveosome), to the ER This pathway bypasses endosomes and the Golgi complex, and is part of the productive infectious route used by SV40

Caveolar endocytosis of simian virus 40 reveals a new two-step vesicular-transport pathway to the ER.

Caveolae 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.

Caveolins, liquid-ordered domains, and signal transduction.

Among the membrane compartments of a cell, vesicles known as "caveolae" have long defied functional characterization. However, since the identification of a family of proteins termed "caveolins", that form and reside in caveolae, a better understanding has emerged. It is now clear that caveolae do not merely play a singular role in the cell, but are pleiotropic in nature-serving to modulate many cellular functions. The purpose of this review is to explicate what is known about caveolins/caveolae and highlight growing areas of caveolar research.

Caveolae: From Cell Biology to Animal Physiology

Co-purification and Direct Interaction of Ras with Caveolin, an Integral Membrane Protein of Caveolae Microdomains DETERGENT-FREE PURIFICATION OF CAVEOLAE MEMBRANES

In simple epithelial cells, apical and basolateral proteins are sorted into separate vesicular carriers before delivery to the appropriate plasma membrane domains. To dissect the putative sorting machinery, we have solubilized Golgi-derived transport vesicles with the detergent CHAPS and shown that an apical marker, influenza haemagglutinin (HA), formed a large complex together with several integral membrane proteins. Remarkably, a similar set of CHAPS-insoluble proteins was found after solubilization of a total cellular membrane fraction. This allowed the cloning of a cDNA encoding one protein of this complex, VIP21 (Vesicular Integral-membrane Protein of 21 kD). The transiently expressed protein appeared on the Golgi-apparatus, the plasma membrane and vesicular structures. We propose that VIP21 is a component of the molecular machinery of vesicular transport.

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M Lehnert

Papers

VIP21, a 21-kD membrane protein is an integral component of trans-Golgi-network-derived transport vesicles.