SELK is a selenoprotein, containing a selenocysteine residue at its active site. SELK is localized to the endoplasmic reticulum and is highly expressed in the heart, where it may function as an antioxidant. SELK can bind to proteins such as the ER-associated degradation (ERAD) components p97 ATPase, Derlin-1 and -2, and SelS as well as proteins in the oligosaccharyltransferase complex, suggesting that SELK may be involved in the Derlin-dependent ERAD of glycosylated misfolded proteins. SELK is also thought to be important in Ca2+ flux in immune cells, and is cleaved by m-calpain in resting macrophages. When these macrophages are activated through different Toll-like receptors, this cleavage is inhibited