Tipping the delicate balance - Defining how proteasome maturation affects the degradation of a substrate for autophagy and endoplasmic reticulum associated degradation (ERAD)

An increasing body of data links endoplasmic reticulum (ER) function to autophagy. Not surprisingly, then, some aberrant proteins in the ER can be destroyed either via ER associated degradation (ERAD), which is proteasome-mediated, or via autophagy. One such substrate is the "Z" variant of the alpha-1 protease inhibitor (A1 Pi), variably known as A1 Pi-Z or ATZ ("anti-trypsin, Z variant"). The wild type protein is primarily synthesized in the liver and is secreted. In contrast, AT-Z, like other ERAD substrates, is retro-translocated from the ER and delivered to the proteasome. However, ATZ can form high molecular weight polymers that are degraded via autophagy, and cells that accumulate ATZ polymers ultimately succumb, which leads to liver disease. Therefore, identifying genes that have an impact ATZ turnover represents an active area of research. To this end, a yeast expression system for ATZ has proven valuable. For example, a recent study using this system indicates that the activity of a proteasome assembly chaperone (PAC) is critical for maximal ATZ turnover, which suggests a new role for PACs. Because PACs are conserved, it will be critical to analyze whether these dedicated chaperones are implicated in other diseases associated with ERAD and autophagy.