ERp29 is a ubiquitously expressed endoplasmic reticulum (ER) protein, which is found in the folding complexes of several secretory proteins in the ER. In our previous work, it was suggested that ERp29 function is critical for the folding/secretion of thyroglobulin (Tg), a major secretory product of thyroid cells. Current work is an attempt to substantiate this assumption by answering the question whether the secretion of Tg can be regulated through the manipulation of ERp29 expression in the FRTL-5 rat thyroid cells. Indeed, transient overexpression of ERp29 resulted in twofold enhancement of the Tg secretion whereas the RNAi-mediated ERp29 silencing led to the attenuation of the Tg export. Mutational analysis has suggested two loci that might be involved in the ERp29-Tg interactions: the interdomain linker including Cys157, an amino acid, which is important for the structural integrity of the C-terminal domain and an uncharged surface on the N-terminal domain flanked by Tyr64 and Gln70. (c) 2005 Elsevier Inc. All rights reserved.