Fig. 3. Effect of cAMP on TTF1 phosphopeptides in FRTL5 thyroid cells. FRTL5
cells were deprived of TSH for 3 days in serum-free medium and
metabolically labeled with [32P]Pi for 4 h in the presence or absence of forskolin (40 µM), a
neutral stimulator of adenylate cyclase. TTF1 was immunoprecipitated
from total cell extracts purified by SDS-PAGE and digested with the
proteolytic enzyme, thermolysin. The resulting peptides were
size-fractionated by two-dimensional TLC, and the label was detected by
autoradiography. Arrows, the peptides a and f that are
stimulated by forskolin. O, origin. A representative set
of three independent experiments that yielded similar results is shown.
An identical phosphopeptide map was observed in cells stimulated with
either TSH (10 milliunits/ml) or cAMP analogues.