Baculovirus-based expression of mammalian caveolin in Sf21 insect cells. A model system for the biochemical and morphological study of caveolae biogenesis.
Caveolae were originally defined morphologically as 50-100 nm noncoated vesicular organelles located at or near the plasma membrane. Caveolin, a vesicular integral membrane protein of 21 kDa, is a principal protein component of caveolae membranes in vivo. Caveolin interacts with itself to form high molecular mass oligomers, suggesting that it might play a structural role in the formation of caveolae membranes. However, it remains controversial whether recombinant expression of caveolin is necessary or sufficient to generate caveolae membranes in vivo. To directly address this issue, we have taken a different experimental approach by exploiting a heterologous expression system. Here, we have recombinantly expressed mammalian caveolin in Sf21 insect cells using baculovirus-based vectors. Two isoforms of caveolin have been identified that differ at their extreme N terminus; alpha-caveolin contains residues 1-178, and beta-caveolin contains residues 32-178. After recombinant expression in Sf21 insect cells, both alpha- and beta-caveolin formed SDS-resistant high molecular mass oligomers of the same size as native caveolin. Morphologically, expression of either caveolin isoform resulted in the intracellular accumulation of a homogeneous population of caveolae-sized vesicles with a diameter between 50 and 120 nm (80.3 +/- 14.8 nm). This indicates that each caveolin isoform can independently generate these structures and that caveolin residues 1-31 are not required for this process. Using caveolin as a marker protein and a detergent-free procedure to purify caveolae from mammalian cells, we purified these recombinant caveolin-induced vesicles from insect cells. These purified recombinant vesicles: (i) have the same buoyant density as mammalian caveolae; (ii) appear as approximately 50-100 nm membranous structures by whole-mount electron microscopy; and (iii) contain approximately 95% of the recombinantly expressed caveolin protein by Western blotting. Immuno-labeling of these structures with anti-caveolin IgG confirmed that they contain caveolin. Thus, ectopic overexpression of caveolin in this heterologous system is sufficient to drive the formation of caveolae-like vesicles. Further functional analysis demonstrated that caveolin was capable of interacting with a known caveolin-interacting protein, Ha-Ras, when coexpressed in insect cells by co-infection with two recombinant baculoviruses. Taken together, our results demonstrate that baculovirus-based expression of caveolin in insect cells provides an attractive experimental system for studying the biogenesis of caveolae.