Bag6 complex contains a minimal tail-anchor–targeting module and a mock BAG domain
BCL2-associated athanogene cochaperone 6 (Bag6) plays a central role in cellular homeostasis in a diverse array of processes and is part of the heterotrimeric Bag6 complex, which also includes ubiquitin-like 4A (Ubl4A) and transmembrane domain recognition complex 35 (TRC35). This complex recently has been shown to be important in the TRC pathway, the mislocalized protein degradation pathway, and the endoplasmic reticulum-associated degradation pathway. Here we define the architecture of the Bag6 complex, demonstrating that both TRC35 and Ubl4A have distinct C-terminal binding sites on Bag6 defining a minimal Bag6 complex. A crystal structure of the Bag6–Ubl4A dimer demonstrates that Bag6–BAG is not a canonical BAG domain, and this finding is substantiated biochemically. Remarkably, the minimal Bag6 complex defined here facilitates tail-anchored substrate transfer from small glutamine-rich tetratricopeptide repeat-containing protein α to TRC40. These findings provide structural insight into the complex network of proteins coordinated by Bag6.
© 2014 National Academy of Sciences. Edited by Gregory A. Petsko, Weill Cornell Medical College, New York, NY, and approved December 1, 2014 (received for review February 12, 2014). Published online before print December 22, 2014, doi: 10.1073/pnas.1402745112. We thank Daniel Lin and Jens Kaiser for help with data processing; Yoko Shibata and Richard Morimoto (Northwestern University) for plasmids; Michael Rome and Meera Rao for critical reading of the manuscript; members of the W.M.C. laboratory for support and useful discussions; the staff at the Advanced Light Source for assistance with synchrotron data collection; and Gordon and Betty Moore for support of the Molecular Observatory at California Institute of Technology. W.M.C. is supported by National Institutes of Health Grant R01GM097572. Author contributions: J.-Y.M., J.W.C., and W.M.C. designed research; J.-Y.M., J.W.C., M.Z., Y.X., and Y.Y. performed research; M.Z., Y.X., and Y.Y. contributed new reagents/analytic tools; J.-Y.M., Y.Y., and W.M.C. analyzed data; and J.-Y.M. and W.M.C. wrote the paper. The authors declare no conflict of interest. This article is a PNAS Direct Submission. Data deposition: Crystallography, atomic coordinates, and structure factors reported in this paper have been deposited in the Research Collaboratory for Structural Bioinformatics Protein Data Bank, www.rcsb.org/pdb/ (accession no. 4WWR). This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1402745112/-/DCSupplemental.
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