The ER is responsible for the folding and posttranslational modification of over a third of all proteins in eukaryotes. Impaired degradation of proteins is strongly linked to neurodegenerative and protein misfolding diseases. Here we examine the ER-associated protein degradation (ERAD) pathway, which governs the extraction of misfolded proteins or misassembled protein complexes from the ER's membrane and lumen and their transport to the cytoplasm where they are degraded by the proteasome. The ERAD is targeted in cancer treatments since cancerous cells require an increased capacity for protein folding and degradation. Two integral membranes proteins that belong to the family of selenoproteins contribute to the ERAD machinery: selenoprotein S (SelS) and selenoprotein K (SelK). Since all selenoproteins are enzymes SelS and SelK are most likely catalytically active but their specific contribution to the ERAD pathway is yet unknown. We recently discovered that SelK is able to cleave its own peptide bond, releasing a selenocysteine?containing peptide, and thus terminating enzymatic activity. We propose that this autoproteolysis is a regulatory mechanism responsible for SelK associations with different membrane complexes. We will characterize the cleavage mechanism, cleavage sites and the unprecedented contribution of selenocysteine to the peptide bond cleavage. We will then examine whether SelK protein partners affect the cleavage rate or sites and whether truncated forms of SelK are able to bind selected protein partners. In a related thrust, we will examine how SelK's protein partner, SelS, coordinates the recruitment of the AAA ATPase valosin-containing protein (VCP) p97 to the membrane channel that translocates misfolded proteins (dislocon). The cytoplasmic p97 provides the energy necessary for pulling misfolded protein out of the dislocon and hence is central to the ERAD process. Because selenoproteins are often found to detoxify or regulate reactive oxidative species we hypothesize that SelS not only recruits p97 but also regulates its ATPase activity and sensitivity to oxidative modifications. We will map SelS interactions with p97 and derlin-1, a transmembrane contributor to the dislocon. Also SelS's ability to interact with additional protein substrates while bound to p97 or derlin-1 will be assessed. The proposed experimental work will unveil the molecular interactions between SelS, SelK, derlin-1, and p97, thus clarifying the steps required for complex assembly of the dislocon and its energy source, p97. In addition, it will be clarified to what extent SelS acts -in a redox state dependent way- as sensor of oxidants and protects p97 from damage. Together, our studies will dramatically advance our understanding of SelS's and SelK's contribution to protein degradation and of the role of their selenocysteine in complex formation and in enzymatic reactions. Because of the specialized chemistry associated with selenocysteine, SelS and SelK present themselves as unique drug targets whose selenium based reactivity can be targeted.
Impaired protein degradation is strongly linked to neurodegenerative and protein misfolding diseases and thus the cellular machinery that monitors and maintains protein fidelity is a prime drug target. Understanding how selenoproteins contribute to the endoplasmic reticulum associated protein degradation pathway will expand our grasp of the steps that link the degradation machineries of the ER membrane and the cytoplasmic ones. These selenoproteins are novel drug targets since their reactive selenocysteine can be targeted to block particular contributions to protein homeostasis.