Immunofluorescence Assay for HA Staining

Immunofluorescence assays were performed as described in ref. ^{56 (link)}. Briefly, for HA staining, purified cells were fixed with 4% paraformaldehyde and 0.05% glutaraldehyde in PBS for 1 h, permeabilised with 0.1% Triton X-100/PBS for 10 min and blocked with 2% BSA/PBS for 2 h. Primary antibodies were diluted in blocking solution (rat anti-HA clone 3F10, 1:1000 from Sigma-Aldrich, reference 000000011867431001). Anti-rat Alexa488 (Life Technologies, reference A-11006) was used as a secondary antibody together with 4′,6-diamidino-2-phenylindole (DAPI) and diluted 1:1000 in blocking solution. Confocal images were acquired with a LSM800 scanning confocal microscope (Zeiss).