Isoforms of caveolin-1 and caveolar structure

The relationship between caveolin-1 isoforms (α and β) and caveolar ultrastructure was studied. By immunofluorescence microscopy of human fibroblasts, caveolae were observed as dots positive for caveolin-1, but many dots labeled by an antibody recognizing both isoforms (anti-αβ) were not labeled by another antibody specific for the α isoform (anti-α). Immunogold electron microscopy of freeze-fracture replicas revealed caveolae of different depths, and indicated that anti-α labeled deep caveolae preferentially over shallow ones, whereas anti-αβ labeled both forms with an equivalent frequency and intensity. The presence of the β isoform in deep caveolae was confirmed by labeling epitope-tagged β-caveolin. When made to be expressed in HepG2 cells lacking endogenous caveolins, the α isoform formed caveolar depressions efficiently, but the β isoform hardly did so. Caveolae were also formed in cells expressing the two isoforms, but their frequency was variable among cells of the same clone. Coexpression of caveolin-1 and caveolin-2 caused more efficient formation of deep caveolae than caveolin-1 alone. The result indicates that the two isoforms of caveolin-1 have a different potential for forming caveolae structure, and more importantly, that deep and shallow caveolae may be diversified in their molecular composition.