Project Abstract. Defects in ER protein homeostasis (proteostasis) perturb the ER proteome and lead to the accumulation of misfolded proteins which is detrimental to the cell. This is underscored by the fact that numerous human diseases from cystic fibrosis to neurodegeneration are caused by defects in ER proteostasis. Historically, the field has focused on how ER protein misfolding is sensed and rectified resulting in the detailed characterization of the unfolded protein response (UPR) and ER-associated degradation (ERAD); pathways that aim to mitigate protein misfolding. However, whether processes exist that prevent ER protein misfolding in the first place were largely unknown. Recent studies indicate the presence of a surveillance system that acts during protein synthesis to tune ER protein translation based on the status of the ER proteome. This process, termed pre-emptive ER quality control relies on local regulation of translation at the ribosome to modulate the biogenesis of ER proteins and ubiquitin dependent degradation of misfolded proteins prior to ER insertion. Therefore, pre- emptive ER quality control oversees translation to prevent folding defects. Because this system is powerful way to maintain a pristine ER proteome without eliciting the UPR or ERAD, mechanisms that activate this process may be pursued to advance therapies for a variety of diseases caused by protein misfolding. However, we do not know the identity of the molecules or mechanisms regulating pre-emptive ER quality control. This level of understanding is critical to develop a framework understanding how ER proteostasis defects cause disease. In ongoing studies in my lab, we have found that UBXN1 is a new modulator of pre-emptive ER quality control. UBXN1 is best studied as an adaptor to the p97 ATPase, a critical player in ERAD and a major research focus of my group. In cells lacking UBXN1 we find a significant upregulation of many ER proteins and robust activation of the unfolded protein response. Our studies suggest that UBXN1 represses translation. Surprisingly, UBXN1 does not function in ERAD and its role in translation is independent of p97. The objective of this proposal is to characterize this new role of UBXN1 as a pre-emptive ER quality control factor. We will test the central hypothesis that UBXN1 interacts with unique ribosomal proteins in a ubiquitin-dependent manner to regulate the specific translation of ER proteins. We will further investigate the pathway through which translation impacts UPR activation. In Aim 1 we will identify the mechanisms by which UBXN1 regulates translation, in Aim 2 we will delineate the pathway through which UBXN1 represses the UPR, and in Aim 3 we determine whether UBXN1 enables selective translation of ER proteins. The proposed work is conceptually innovative as we do not fully understand how preemptive ER quality control is regulated. This work is of significance because we will define how UBXN1 maintains ER proteostasis b...