Selenium (Se) is an essential micronutrient important for many aspects of human health, including optimal immune responses. The biological effects of Se are exerted mainly through its incorporation into selenoproteins as the amino acid, selenocysteine (Sec). Twenty-five selenoproteins have been identified in humans, all but one of which also exist as Sec-containing proteins in mice and rats. One selenoprotein that we have shown to be important for optimal immune responses is the endoplasmic reticulum (ER) transmembrane selenoprotein K (Selk). Selk expression is expressed highest in immune cells and is sensitive to levels of Se intake. The main goal of our first funding period was to understand how Selk regulates immune cell function and to use our novel Selk-/- mouse model to determine the role of Selk for in vivo immunity. We were successful in uncovering a role for Selk in generating the calcium (Ca2+) flux that occurs during receptor-mediated activation in immune cells including T and B cells, neutrophils, and macrophages. Our new data have revealed that Selk deficiency leads to impaired Ca2+ flux due to a defect in palmitoylation of the Ca2+ channel protein, inositol-1,4,5-triphosphate receptor (IP3R), in the ER membrane of immune cells. IP3R-driven Ca2+ flux plays a central role in immune cell activation and Selk deficiency was shown in our studies to influence a wide variety of immune responses including anti-viral immunity, peritonitis, and atheroschlerosis. These findings have led to our central hypothesis that Selk functions as a cofactor for the palmitoyl acyl transferase, DHHC6, thereby promoting the palmitoyation of IP3R and other cellular proteins and this serves as a mechanism by which levels of dietary Se regulate immunity. Our innovative studies will definitively determine the role of Selk in the palmitoylation of the IP3R and other proteins and how this affects immune cell function. We also will take an important step toward translating our findings to human health by investigating how dietary Se and genetic polymorphisms are related to Selk expression and immune cell function in humans. We will accomplishment of the following specific aims: Specific Aim 1. Determine the mechanisms by which Selk regulates palmitoylation of the IP3R. Specific Aim 2. Identify cellular proteins that require Selk for palmitoylation and determine how palmitoylation regulates their function. Specific Aim 3. To determine how Selk expression is regulated in human T cells and how this affects IP3R expression and T cell function.