Inhibits the chaperone activity of HSP70/HSC70 by promoting substrate release. Has anti-apoptotic activity. Defects in BAG3 are the cause of myopathy myofibrillar type 6 (MFM6). A neuromuscular disorder that results in early-onset, severe, progressive, diffuse muscle weakness associated with cardiomyopathy, severe respiratory insufficiency during adolescence, and a rigid spine in some patients. At ultrastructural level, muscle fibers display structural alterations consisting of replacement of the normal myofibrillar markings by small, dense granules, or larger hyaline masses, or amorphous material. Defects in BAG3 are the cause of cardiomyopathy dilated type 1HH (CMD1HH). CMD1HH is a disorder characterized by ventricular dilation and impaired systolic function, resulting in congestive heart failure and arrhythmia. Patients are at risk of premature death. Note: This description may include information from UniProtKB.