K. Tanaka et al., Thyrotropin receptor cleavage at site 1 involves two discontinuous segments at each end of the unique 50-amino acid insertion, J BIOL CHEM, 274(4), 1999, pp. 2093-2096
Among the glycoprotein hormone receptors, only the thyrotropin receptor (TS
HR) cleaves (at two sites) into disulfide-linked A and B subunits, A 50-ami
no acid insertion unique to the TSHR ectodomain (residues 317-366) plays no
role in ligand binding or signal transduction, but its deletion abrogates
cleavage at Site 1, closely upstream of the insertion. We sought to define
the region within the 50-amino acid tract involved in TSHR cleavage at Site
1. Mutation of small segments within this region previously failed to prev
ent cleavage at Site 1., We, therefore, divided the 50-amino acid insertion
into quartiles and deleted each one individually (TSHR residues 317-327, 3
28-338, 339-350, and 351-362), As determined by covalent cross-linking of I
-125-TSH to, intact cells expressing the mutant receptors, none of these de
letions prevented TSHR cleavage at Site 1., Neither did larger deletions of
quartiles 1 + 2, 2 + 3, and 3 + 4. However, qualitative differences in the
extent of receptor cleavage suggested that quartiles 1 and 4 were playing
a greater role in cleavage at Site 1 than were the middle two quartiles, In
support of this hypothesis, deletion of these two discontinuous segments a
lmost completely eliminated TSHR cleavage at Site 1.
In conclusion, intramolecular cleavage at Site 1 requires the presence of t
he N-terminal and C-terminal quartiles of the 50-amino acid insertion uniqu
e to the TSHR. Taken together with previous observations, our data suggest
that this tract may provide a discontinuous binding site for a protease tha
t clips the TSHR at Site 1.