To confirm data of the pull-down experiments, coimmunoprecipitation experiments were performed using NIH3T3 or MDCK cells after transfection of pEP24c, an expression vector containing M2 PR/8 [29] or a vector harbouring fusion protein of M2 with fluorescent marker EGFP. 24 h later cell lysates were prepared in the presence of octylglucoside, a detergent that disrupt lipid rafts, as described previously [13] . In the first series of experiments, polyclonal anti-Cav-1 antibodies were used to pull-down Cav-1 complexes from lysates. Precipitated complexes were probed for the presence of M2 after Western Blot and immunostaining. In these experiments, the Cav-1 antibodies clearly pulled down a complex that contained a M2 from pEP24c transfected MDCK cells (Fig. 5B, a) or the M2 fusion protein from pM2PR8-EGFP transfected MDCK or NIH3T3 cells (Fig. 5B, b) as well as infected MDCK cells (Fig. 5B, c left panel) . In the second experimental setting, vice versa, monoclonal anti-EGFP antibodies were used to precipitate M2 binding partners and a rabbit anti-Cav-1 antibody was used to probe for the presence of caveolin (Fig 5B, c right panel) . These types of experimental settings identified M2 complexed with Cav-1 and vice versa in both cell lines, NIH3T3 and MDCK. Thus, the results suggest that M2 has the capability to interact directly or indirectly with caveolin-1 in different cell lines. With respect to the type of interaction, it is notable, that caveolin-1 as well as M2 have been reported to bind cholesterol via cholesterol specific recognition domains [30, 31] . This prompted us to investigate, whether cholesterol is involved in the M2/caveolin-1 interaction. For that purpose methyl-β-cyclodextrin (MβCD) was used to deplete cell lysates from cholesterol before co-immunoprecipatation (Fig. 5B b and 5c) . Interestingly, in pM2PR8-EGFP transfected NIH3T3 cells as well as in PR8 virus-infected MDCK cells, signals from co-immunoprecipated proteins decreased to a certain extent, if cholesterol was removed from the lysate before pull-down. These findings imply, that cholesterol seems to support the interaction of M2 with caveolin-1.