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Abstract:
Lipid rafts are liquid ordered platforms that dynamically
compartmentalize membranes. Caveolins and flotillins
constitute a group of proteins that are enriched in these
domains. Caveolin-1 has been shown to be an essential
component of caveolae. Flotillins were also discovered
as an integral component of caveolae and have since
been suggested to interact with caveolins. However, flotillins
are also expressed in non-caveolae-containing
cells such as lymphocytes and neuronal cells. Hence, a
discrepancy exists in the literature regarding the caveolin
dependence of flotillin expression and their subcellular
localization. To address this controversy, we used mouse
embryonic fibroblasts (MEFs) from caveolin-1 knockout
(Cav-1-/-) and wild-type mice to study flotillin expression
and localization. Here we show that both membrane
association and lipid raft partitioning of flotillins are not
perturbed in Cav-1-/- MEFs, whereas membrane targeting
and raft partitioning of caveolin-2, another caveolin family
protein, is severely impaired. Moreover, we demonstrate
that flotillin-1, but not flotillin-2, associates with lipid
droplets upon oleic acid treatment and that this association
is completely independent of caveolin. Taken together,
our results show that flotillins are localized in lipid rafts
independent of caveolin-1 and that translocation of flotillin-
1 to lipid droplets is a caveolin-independent
process.
