For immunoprecipitation, protein-G-Sepharose was pre-cleared by rotation in 5% (w/v) BSA in TBS for 1 h at 4uC, prior to washing 3 times with protein binding buffer (50 mM Tris-HCl pH 7, 50 mM NaCl, 1 mM ZnSO 4 ). Samples were incubated with monoclonal anti-b1 integrin antibody (4uC, overnight), pre-cleared protein-G-Sepharose was then added and samples rotated (2 h, 4uC). Bound samples were eluted by heating to 85uC with 1xSDS-PAGE sample buffer. Sepharose beads were pelleted and the eluted supernatant heated to 95uC with b-mercaptoethanol. Where cells had been crosslinked, crosslinking was denatured by heating with DTT before elution (30 min, 37uC).