Fig. 6. (A) Surface delivery of newly synthesized wild-type and mutant FcRn chains. Cells were pulsed with ³⁵S-labeled Met+Cys. After the chase times shown, cells were biotinylated at either the apical or the basolateral surface. FcRn was immunoprecipitated from lysates and biotinylated receptors were re-precipitated with streptavidin-agarose. Precipitated proteins were resolved on polyacrylamide gels, which were then dried and exposed to PhosphorImager screens. Arrowheads indicate the immature (i) and mature (m) forms of FcRn chain. (B) Evaluation of the biotinylation of an intracellular protein. Following pulse-labeling and biotinylation without chase, actin was immunoprecipitated from cell lysates and biotinylated actin was re-precipitated as above. The amount of total actin loaded on the gel represents 0.2% of the cell lysate, whereas the amount of biotinylated actin represents 50% of the cell lysate.