The Cholinesterase-like Domain is Required for Folding and Secretion of Thyroglobulin.

Abstract

Thyroid hormone synthesis requires secretion of thyroglobulin, a precursor protein comprising Cys-rich regions I, II, and III (referred to collectively as regions I-II-III) followed by a cholinesterase-like (ChEL) domain. Secretion of mature thyroglobulin requires extensive folding and oligomerization in the ER. Multiple reports have linked mutations in the ChEL domain to thyroid diseases in humans and other animals; these mutations block thyroglobulin from exiting the ER and induce ER stress. The principal hypothesis in this thesis is that thyroglobulin requires the ChEL domain to form its native tertiary and quaternary structure. We report that, in a cell culture system, thyroglobulin with misfolding mutations in the ChEL domain also impairs folding in other domains. A truncated thyroglobulin (regions I-II-III) devoid of the ChEL domain is incompetent for folding and cellular export. However, co-expression of a secretory ChEL domain (which is efficiently secreted) with the truncated thyroglobulin rescued secretion of regions I-II-III by promoting their oxidative maturation and folding. The data indicate that the ChEL domain functions as an intramolecular chaperone. Moreover, a functional ChEL domain engineered to be retained intracellularly co-retains truncated thyroglobulin even as it facilitates the oxidative maturation of regions I-II-III. This indicates that the ChEL domain is also employed as a molecular escort for thyroglobulin. Thyroglobulin becomes a homodimer within the endoplasmic reticulum. The ChEL domain sequence contains predicted helical structures that are homologous to the dimerization helices of acetylcholinesterase. I have found that the ChEL domain can dimerize with itself, just as acetylcholinesterase does, and this is likely to drive the dimerization of wild-type thyroglobulin. Insertion of an N-linked glycan into one of the putative dimerization helices blocks detectable homodimerization of the isolated ChEL domain. Interestingly, co-expression of upstream regions of thyroglobulin, I-II-III, either in cis or in trans, overrides the dimerization defect of such a mutant. These data suggest that intermonomer interactions of the ChEL domain of thyroglobulin are enhanced by additional interactions with upstream regions of thyroglobulin. I conclude that the ChEL domain is required to form a native structure of Tg which is essential for export from the endoplasmic reticulum, and for thyroid hormone.