The Bag6 Complex: Biological Complexity through Modularity

Citation

Mock, Jee Young (2017) The Bag6 Complex: Biological Complexity through Modularity. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/Z91C1TXQ. https://resolver.caltech.edu/CaltechTHESIS:06052017-184553539

Abstract

Proper synthesis and targeting of membrane proteins that contain hydrophobic transmembrane domains are mediated by chaperones and targeting factors. Tail-anchored (TA) proteins are a special class of membrane proteins that are characterized by a single carboxy (C) terminal helix that anchors them to biological membranes. Fungal Guided Entry of Tail-anchored protein (GET) pathway components, which include four soluble proteins—Sgt2, Get3, Get4, Get5—and two membrane bound receptors—Get1 and Get2—mediate TA biogenesis. These proteins maintain TA protein solubility in the aqueous cytosol and target TA to the endoplasmic reticulum. While most of the components are conserved in metazoans, one additional protein, Bag6, reorganizes the sorting complex from the heterotetrameric Get4-5 to the heterotrimeric Bag6-TRC35-Ubl4A. To understand the molecular architecture and mechanism of the Bag6 complex, we took a multidisciplinary approach that combines x-ray crystallography, biochemical reconstitution, and cell biology. Our studies demonstrate that the BAG domain of Bag6 is not a canonical BAG domain. Instead, main role of the Bag6 'mock' BAG domain is to dimerize with Ubl4A. Furthermore, the truncated Bag6 complex defined in this study is sufficient to facilitate substrate transfer from SGTA to TRC40. Lastly, our results unequivocally establish TRC35 as a cytoplasmic retention factor for Bag6. These results provide structural, biochemical and cell biological bases for modular Bag6 function and regulation of nucleocytoplasmic distribution of Bag6 by TRC35.

Thesis Files