The aim of this project was to determine which viral and host proteins bind to purified recombinant human GTPase Rab11a from lysates of HEK-293T cells infected with influenza A/WSN/33 (H1N1) virus. To this aim we used a GST pull-down assay. Approximately 8∙106 HEK-293T cells were infected with influenza A/WSN/33 (H1N1) virus at a multiplicity of infection of 5 (MOI=5) or mock infected. Twelve hours (h) post-infection cells were harvested by centrifugation at 1,000 rpm for 5 min and lysed for 1 h in lysis buffer [50 mM HEPES-NaOH pH 7.5, 200 mM NaCl, 0.5 % (v/v) IGEPAL CA-630 (Sigma-Aldrich), 1 mM β-mercaptoethanol, 1x PMSF, 1x protease inhibitor (Roche, complete mini, EDTA-free)] at 4 °C. Supernatants were collected by centrifugation at 13,000 rpm for 5 min at 4 °C. Approximately 0.2 mg of purified GST-tagged Rab11CA (constitutively active Rab11a with a Q70L substitution) or GST tag was bound to pre-equilibrated Glutathione Sepharose 4B beads (GE Healthcare) (100 µl slurry per 1 ml sample volume) at 4 °C for 3 h in binding buffer [50 mM HEPES-NaOH pH 7.5, 300 mM NaCl, 10 % (v/v) glycerol, 8 mM MgCl2, 10 µM GTP-γ-S (Abcam)]. GST-Rab11CA or GST tag bound beads were incubated with virus infected or mock infected cell lysates for 1 h at 4 °C followed by five washes in binding buffer. Rab11CA and bound proteins were released in the same buffer supplemented with 1 mM DTT and 0.1 mg human rhinovirus (HRV) 3C protease to cleave off Rab11CA from the GST tag. After 1 h incubation at 4 °C released proteins were cleared from the beads by centrifugation at 1,000 rpm for 5 min at 4 °C. GST pull-down protein fractions were used for analysis by mass spectrometry.