Cells were transiently transfected with the indicated constructs and incubated with ¹²⁵I-hCG for 2 h at 37°C. At this time (t = 0), the cells were washed with a neutral buffer (to remove the free hormone) and briefly treated with an isotonic pH 3 buffer (to remove the surface-bound hormone). They were then incubated for an additional 2 h at 37°C in medium containing an excess of nonradioactive hCG. At the end of this incubation, the medium was collected and used to measure degraded and undegraded hormone, and the cells were used to determine the amount of radioactivity that remained cell associated. The radioactivity present in each of these three “compartments” is expressed as a percentage of all the radioactivity found. After the acid release, at the beginning of the second 2-h incubation, more than 95% of the total radioactivity found was cell associated.

• Each value represents the mean ± S.E.M. of three independent transfections.