BAG6/BAT3: emerging roles in quality control for nascent polypeptides.

BAG6 (also known as BAT3/Scythe) is a ubiquitin-like protein that is thought to participate in a variety of seemingly unrelated physiological and pathological processes, such as apoptosis, antigen presentation and the T-cell response. Recent studies have shown that BAG6 is essential for the quality control of aggregation-prone polypeptide biogenesis. It forms part of a complex that determines the fate of newly synthesized client proteins for membrane insertion, ubiquitin-mediated degradation and/or aggregate formation. A biologically relevant transmembrane protein family has recently been shown to be a major client of BAG6, suggesting that many of the known diverse BAG6 functions can be interpreted by BAG6-mediated control of membrane protein biogenesis. In this review, we summarize the current understanding of the physiological roles of BAG6 with a particular focus on quality control for nascent chain polypeptides.