Background: The BAG6 complex resides in the cytosol and acts as a sorting point
to target diverse hydrophobic protein substrates along their appropriate paths,
including proteasomal degradation and ER membrane insertion. Composed of a
trimeric complex of BAG6, TRC35 and UBL4A, the BAG6 complex is closely
associated with SGTA, a co-chaperone from which it can obtain hydrophobic
substrates.
Methodology and Principal Findings: SGTA consists of an N-terminal
dimerisation domain (SGTA_NT), a central tetratricopeptide repeat (TPR) domain,
and a glutamine rich region towards the C-terminus. Here we solve a solution
structure of the SGTA dimerisation domain and use biophysical techniques to
investigate its interaction with two different UBL domains from the BAG6 complex.
The SGTA_NT structure is a dimer with a tight hydrophobic interface connecting two
sets of four alpha helices. Using a combination of NMR chemical shift perturbation,
isothermal titration calorimetry (ITC) and microscale thermophoresis (MST)experiments we have biochemically characterised the interactions of SGTA with
components of the BAG6 complex, the ubiquitin-like domain (UBL) containing
proteins UBL4A and BAG6. We demonstrate that the UBL domains from UBL4A
and BAG6 directly compete for binding to SGTA at the same site. Using a
combination of structural and interaction data we have implemented the HADDOCK
protein-protein interaction docking tool to generate models of the SGTA-UBL
complexes.
Significance: This atomic level information contributes to our understanding of the
way in which hydrophobic proteins have their fate decided by the collaboration
between SGTA and the BAG6 complex.