Trypsin-sensitivity of cell/matrix-associated FBG. A549 cells were metabolically labeled with ³⁵S-cys + ³⁵S-met for 10 (lanes 1), 20 (lanes 2), 30 (lanes 3), 60 (lanes 4), 90 (lanes 5), and 120 (lanes 6) minutes. FBG secreted into the media was sampled at 30, 60, and 120 minutes (left panel); total FBG remaining cell-associated (middle panel) and intracellular FBG remaining after trypsinization of cell monolayers (right panel) were sampled at all time points, immunopurified, and resolved under nonreducing SDS-PAGE conditions. The position of migration of secreted and cell-bound intact FBG (disulfide-bonded Aα, Bβ, and γ chains) is denoted at 340 kD. The intracellular intermediates of FBG were determined by two-dimensional gel electrophoresis (not shown) and are labeled accordingly.