Thyroglobulin (Tg) is a large glycoprotein specific to the thyroid gland and
it is the precursor of the iodinated thyroid hormones thyroxine (T4) and
triiodothyronine (T3). The N-terminal section of Tg contains 11 repeats of a
domain of about 70 amino acids which is known as the Tg type-1 repeat [1,2,3].
Such a domain has also been found (as a single or repeated sequence) in the
proteins listed below:
In the HLA class II associated invariant chain . In that protein the
type-1 repeat is encoded by an exon which is alternatively spliced and is
only present in a longer form of the protein.
In human pancreatic carcinoma marker proteins GA733-1 and GA733-2 (which is
also known as epithelial cell surface antigen (EPG) or adenocarcinoma-
associated antigen (KSA) or antigen KS 1/4). These proteins may function as
growth factor receptors .
In nidogen (entactin), a sulfated glycoprotein which is widely distributed
in basement membranes and that is tightly associated with laminin. Nidogen
probably has a role in cell-extracellular matrix interactions.
In the insulin-like growth factor binding proteins (IGFBP)  (see
In saxiphilin, a transferrin-like protein from frog that binds specifically
to the neurotoxin saxitoxin .
In equistatin, a thiol-protease inhibitor from sea anemone .
The domain contains six cysteines (Tg type-1a) involved in 3 disulfide bonds
(see <PDB:1ZT3>) , but also exists in a variant that lacks the third and
fourth cysteines (Tg type-1b). One Gln residue is strongly conserved.
Schematic representations of the two Tg type-1 domains are shown below:
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