Secretion of a low-molecular-weight species of endogenous GRP94 devoid of the KDEL motif during endoplasmic reticulum stress in Chinese hamster ovary cells

GRP94 (glucose-regulated protein 94) is a well-studied chaperone with a lysine, aspartic acid, glutamic acid and leucine (KDEL) motif at its C-terminal, which is responsible for GRP94 localization in the endoplasmic reticulum (ER). GRP94 is upregulated during ER stress to help fold unfolded proteins or direct proteins to ER-associated degradation. In a previous study, engineered GRP94 without the KDEL motif stimulated a powerful immune response in vaccine cells. In this report, we show that endogenous GRP94 is naturally secreted into the medium in a truncated form that lacks the KDEL motif in Chinese hamster ovary cells. The secretion of the truncated form of GRP94 was stimulated by the induction of ER stress. These truncations prevent GRP94 recognition by KDEL receptors and retention inside the cell. This study sheds light on a potential trafficking phenomenon during the unfolded protein response that may help understand the functional role of GRP94 as a trafficking molecule.