Isoforms of caveolin-1 and caveolar structure

The relationship between caveolin-1 isoforms (alpha and beta) and caveolar ultrastructure was studied, By immunofluorescence microscopy of human fibro blasts, caveolae were observed as dots positive for caveolin-1, but many do ts labeled by an antibody recognizing both isoforms (anti-alpha beta) were not labeled hy another antibody specific for the alpha isoform (anti-alpha) , Immunogold electron microscopy of freeze-fracture replicas revealed caveo lae of different depths, and indicated that anti-ct labeled deep caveolae p referentially over shallow ones, whereas anti-ap labeled both forms with an equivalent frequency and intensity, The presence of the beta isoform in de ep caveolae was confirmed by labeling elpitope-tagged beta -caveolin When m ade to be expressed in HepG2 cells lacking endogenous caveolins, the alpha isoform formed caveolar depressions efficiently, but the beta isoform hardl y did so. Caveolae were also formed in cells expressing the two isoforms, b ut their frequency was variable among cells of the same clone. Coexpression of caveolin-1 and caveolin-2 caused more efficient formation of deep caveo lae than caveolin-1 alone, The result indicates that the two isoforms of ca veolin-1 have a different potential for forming caveolae structure, and mor e importantly, that deep and shallow caveolae may be diversified in their m olecular composition.