Figure 3.

CFTR is an integral membrane protein. (A) ER-derived microsomes were prepared from CFTR-expressing cells, extracted with sodium carbonate, and then centrifuged. Total protein from the membrane pellet (P) and supernatant (S) was resolved by SDS-PAGE and subjected to immunoblot analysis. BiP and Sec61p serve as soluble and membrane protein controls, respectively. (B) Cell extracts from CFTR-expressing wild-type and pre1-1pre2-2 cells were prepared, layered at the bottom of a centrifuge tube, and subjected to ultracentrifugation as described in MATERIALS AND METHODS. Immunoblots to detect the indicated proteins in fractions from the top to the bottom of the gradient are shown. The position in the gradient at which the cell extract was loaded is also indicated.