K. Tanaka et al., The shed thyrotropin receptor is primarily a carboxyl terminal truncated form of the A subunit, not the entire A subunit, MOL C ENDOC, 150(1-2), 1999, pp. 113-119
The TSH receptor (TSHR) sheds its A subunit, particularly when cells are cu
ltured in serum-poor medium. This shed A subunit is reported to be smaller
than the cell-associated receptor because of the loss of glycan without cha
nge in its polypeptide core. Contrary to previous deductions, we now find t
hat the 'small' shed A subunit has lost a glycan moiety because of the prot
eolytic clipping of a small C-terminal fragment containing an Asn-linked gl
ycan. Moreover, this lost peptide fragment contains cysteine residues likel
y involved in A. subunit linkage to the membrane-associated B subunit. Prog
ressive lowering of the serum concentration in culture medium accentuates t
he process. Therefore,'small' A subunit shedding does not appear related to
a physiological mechanism involving disulfide bond reduction. On the other
hand, we detected, for the first time, shedding of a lesser amount of norm
al-sized, in addition to small, A subunits, especially by cells cultured in
standard serum concentrations. (C) 1999 Elsevier Science Ireland Ltd. All
rights reserved.