This domain is found in large proline-rich protein BAG6 protein. It has a conserved SDL sequence motif. Human BAG6 is an ATP-independent molecular chaperone preventing the aggregation of misfolded and hydrophobic patches-containing proteins []. It functions as part of a cytosolic protein quality control complex, the BAG6/BAT3 complex, which maintains these client proteins in a soluble state and participates to their proper delivery to the endoplasmic reticulum or alternatively can promote their sorting to the proteasome where they undergo degradation [, , , ].
This is the C-terminal domain of Ubiquitin-like protein 4A, an orthologue of yeast Get5. In budding yeasts, Get proteins directly mediate the insertion of newly synthesized TA proteins into endoplasmic reticulum membranes. Similarly, mammalian BAG6, Ubl4a, and SGTA make up a trimeric complex that binds TA proteins post-translationally and then loads them onto the cytosolic ATPase TRC40, which in turn targets them to the endoplasmic reticulum. Structural studies show that this C-terminal TUGS domain of Ubl4a is essential for BAG6 tethering. Given that BAG6 mediates oligomeric complex formation of Ubl4a, TRC35, and TRC40 (mammalian counterparts of Get5, Get4, and Get3, respectively), the C-terminal TUGS domain might be crucial for supporting BAG6-mediated Ubl4a-TRC35 complexformation in humans as an alternative to the direct Get5-Get4 interaction in yeast [, ].
