SUMMARY The precise role of caveolae, the characteristic plasma membrane invaginations present in many cells, still remains debated. The high density of caveolae in cells experiencing mechanical stress led us to investigate their role in membrane-mediated mechanical response. Acute mechanical stress induced by cell osmotic swelling or by uniaxial stretching results in the immediate disappearance of caveolae, which is associated with a reduced caveolin/Cavin1 interaction, and an increase of free caveolins at the plasma membrane. Tether pulling force measurements in live cells and in plasma membrane spheres demonstrate that caveola flattening and disassembly is the primary actin and ATP-independent cell response which buffers membrane tension surges during mechanical stress. Conversely, stress release leads to complete caveola reassembly in an actin and ATP-dependent process. The absence of a functional caveola reservoir in myotubes from muscular dystrophic patients enhanced membrane fragility under mechanical stress. Our findings support a new role for caveolae as a physiological membrane reservoir that allows cells to quickly accommodate sudden and acute mechanical stresses.

Caveolae are abundant cell-surface organelles involved in lipid regulation and endocytosis. We used comparative proteomics to identify PTRF (also called Cav-p60, Cavin) as a putative caveolar coat protein. PTRF-Cavin selectively associates with mature caveolae at the plasma membrane but not Golgi-localized caveolin. In prostate cancer PC3 cells, and during development of zebrafish notochord, lack of PTRF-Cavin expression correlates with lack of caveolae, and caveolin resides on flat plasma membrane. Expression of PTRF-Cavin in PC3 cells is sufficient to cause formation of caveolae. Knockdown of PTRF-Cavin reduces caveolae density, both in mammalian cells and in the zebrafish. Caveolin remains on the plasma membrane in PTRF-Cavin knockdown cells but exhibits increased lateral mobility and accelerated lysosomal degradation. We conclude that PTRF-Cavin is required for caveola formation and sequestration of mobile caveolin into immobile caveolae.

Polymerase I and transcript release factor (PTRF)/Cavin is a cytoplasmic protein whose expression is obligatory for caveola formation. Using biochemistry and fluorescence resonance energy transfer–based approaches, we now show that a family of related proteins, PTRF/Cavin-1, serum deprivation response (SDR)/Cavin-2, SDR-related gene product that binds to C kinase (SRBC)/Cavin-3, and muscle-restricted coiled-coil protein (MURC)/Cavin-4, forms a multiprotein complex that associates with caveolae. This complex can constitutively assemble in the cytosol and associate with caveolin at plasma membrane caveolae. Cavin-1, but not other cavins, can induce caveola formation in a heterologous system and is required for the recruitment of the cavin complex to caveolae. The tissue-restricted expression of cavins suggests that caveolae may perform tissue-specific functions regulated by the composition of the cavin complex. Cavin-4 is expressed predominantly in muscle, and its distribution is perturbed in human muscle disease associated with Caveolin-3 dysfunction, identifying Cavin-4 as a novel muscle disease candidate caveolar protein.

In mammalian cells three closely related cavin proteins cooperate with the scaffolding protein caveolin to form membrane invaginations known as caveolae. Here we have developed a novel single-molecule fluorescence approach to directly observe interactions and stoichiometries in protein complexes from cell extracts and from in vitro synthesized components. We show that up to 50 cavins associate on a caveola. However, rather than forming a single coat complex containing the three cavin family members, single-molecule analysis reveals an exquisite specificity of interactions between cavin1, cavin2 and cavin3. Changes in membrane tension can flatten the caveolae, causing the release of the cavin coat and its disassembly into separate cavin1-cavin2 and cavin1-cavin3 subcomplexes. Each of these subcomplexes contain 9 ± 2 cavin molecules and appear to be the building blocks of the caveolar coat. High resolution immunoelectron microscopy suggests a remarkable nanoscale organization of these separate subcomplexes, forming individual striations on the surface of caveolae.DOI: http://dx.doi.org/10.7554/eLife.01434.001

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The caveolar membrane microdomain plays an integral role in stabilizing the muscle fiber surface in mice and zebrafish.

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Phosphocaveolin-1 regulates a positive feedback loop that responds to mechanical stress to induce caveola biogenesis by relieving Egr1 transcriptional inhibition of caveolin-1 and cavin-1.