PROJECT SUMMARY
The secretory pathway must fold and traffic one third of the proteome, while handling an
exceptional dynamic range of client load. Quality control mechanisms prevent proteostasis in the
secretory pathway from being overwhelmed by proteins in non-native states. One such
mechanism is preemptive quality control (preQC), wherein ER stress inhibits ER translocation of
nascent proteins. This process protects the ER from being overwhelmed with nascent protein
during misfolded protein stress, a situation associated with diverse disease included diabetes and
neurodegenerative diseases. However, this protection leads to secretory protein accumulating in
the cytosol, which can be proteotoxic and lead to cell death and dysfunction. Due to
methodological limitations, the substrates, causes, mechanism, and consequences of preQC are
largely unknown. Hence, the physiological relevance of preQC, while expected to be significant,
is not well understood. We propose to apply our recently developed assay for secretory protein
mistargeting to systematically characterize preQC. We will identify which stresses induce preQC
for a series of protein substrates in secretory cells from diverse human lineages. We will identify
what factors govern triaging of mistargeted secretory proteins between aggregation and
degradation mechanisms. Because our assay is performed in living cells, we will determine which
signaling pathways mediate induction of preQC by ER stress. Finally, we will extend our assay to
allow proteome-wide quantification of both basal and stress-dependent ER mistargeting. This
extension will allow us to identify the factors that govern susceptibility of proteins to preQC. The
outcome of the proposed research will be systematic characterization of how the cell remodels
ER translocation in response to stress.