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.

Quantitative ultrastructural analysis and proteomics detail CLIC structure, composition, and function.

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.

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