Roles for Hsp70 and Hsp40 in Membrane Protein Triage

Univ Of North Carolina Chapel Hill, Chapel Hill NC

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Abstract

? DESCRIPTION (provided by applicant): Study of protein misfolding associated with Cystic Fibrosis, Hypogonadism, Immune Deficiency, and Neurodegenerative disease by the Cyr group is revealing mechanisms for protein triage that are essential for organismal viability. This proposal focuses on understanding quality control (QC) of membrane proteins (MP) in the endoplasmic reticulum (ER). Approximately 30% of total protein synthesis occurs in the ER with defects in MP biogenesis resulting from natural inefficiencies in folding, missense mutations, and ER-stress. ABC-Transporters, P-Type ATPases and G-protein coupled receptors contain very large cytoplasmic domains, so their folding requires coordinated action of ER-associated and cytosol molecular chaperones. We find that misfolded forms of the same MP accumulate in globally misfolded, partially misfolded and aggregated states, which necessitates employment of conformation specific triage mechanisms. The ER- transmembrane Hsp40 JB12 is identified as a triage factor recruits Hsp70 to the cytoplasmic face of the ER for it to mediate the delivery of globally misfolded MPs to ERAD machinery. Remarkably, JB12/Hsp70 senses accumulation of partially folded and ERAD-resistant MPs to initiate a site-specific induction of ERQC- autophagy for their disposal. Function of JB12/Hsp70 in ERQC is critical because the sequestration of JB12 by pools of aggregated membrane proteins leads to rapid induction of ER-stress induced apoptosis. The proposed studies will identify new mechanisms for triage of MPs and define nodes for crosstalk between the Hsp70 system, ERAD machinery, and the autophagy pathway. The study design incorporates use of model cells systems and cultures of primary lung cells from Cystic Fibrosis patients. This is approach permits the translation of basic finding to uncover mechanisms for protein homeostasis in patients with a terminal lung disease.