Figure 5.

Degradation of Tf bound to wild-type TR and TR TM mutants. Equivalent cell numbers of CEFs expressing wild-type TR, Ii_TM 11–19, TR_TM Thr¹¹Gln¹⁷Thr¹⁹, and Ii_TM 11–19 Ala¹¹Ala¹⁷Ala¹⁹ were preincubated in serum-free medium for 30 min at 37°C and then incubated with ¹²⁵I-labeled Tf in BSA-PBS for 10 min. at 37°C. The cells were then washed and reincubated at 37°C in DMEM containing 50 μg/ml unlabeled Tf and 0.1% BSA for various times. Acid-soluble radioactivity (○) or acid-insoluble ¹²⁵I-labeled Tf (▵) released into the medium, as well as surface bound Tf (acid, □) and internalized (base, ⋄) ¹²⁵I-labeled Tf was determined as described under MATERIALS AND METHODS and is expressed as a percentage of total radioactivity recovered. Each data point is the mean ± SE from an experiment done in triplicate. A representative experiment of three is shown.