Maintaining the dynamic proteome of a living cell in the face of an 
ever-changing environment depends on a fine-tuned balance of protein synthesis 
and protein degradation. Molecular chaperones exert key functions during protein 
homeostasis (proteostasis). They associate with nonnative client proteins 
following synthesis or damage and facilitate client sorting and folding. When 
client proteins are terminally misfolded, chaperones cooperate with protein 
degradation systems to dispose of such clients. This dual proteostasis activity 
of chaperones is essential for maintaining cell function under normal growth 
conditions and becomes even more important under stress conditions such as heat 
and oxidative stress. The recent identification of chaperone-assisted selective 
autophagy (CASA) as a tension-induced autophagy pathway highlights the critical 
role of molecular chaperones in mechanically strained cells and tissues. The 
CASA complex, assembled by the cochaperone BAG3, coordinates protein degradation 
and protein synthesis in response to mechanical force. Here we describe the 
composition and function of this chaperone complex in mammals and discuss its 
relevance for tissue homeostasis and the regulation of cell adhesion, migration 
and proliferation. We provide a unifying concept for the function of BAG3, which 
integrates its involvement in muscle maintenance, tumor formation and virus 
infection.