QUALITY-CONTROL OF ER SYNTHESIZED PROTEINS - AN EXPOSED THIOL-GROUP AS A 3-WAY SWITCH MEDIATING ASSEMBLY, RETENTION AND DEGRADATION

Plasma cells secrete IgM only in the polymeric form: the C-terminal cy steine of the mu heavy chain (Cys575) is responsible for both intracel lular retention and assembly of IgM subunits. Polymerization is not qu antitative, and part of IgM is degraded intracellularly. Neither chlor oquine nor brefeldin A (BFA) inhibits degradation, suggesting that thi s process occurs in a pre-Golgi compartment. Degradation of IgM assemb ly intermediates requires Cys575: the monomeric IgMala575 mutant is st able also when endoplasmic reticulum (ER) to Golgi transport is blocke d by BFA. Addition of the 20 C-terminal residues of mu to the lysosoma l protease cathepsin D is sufficient to induce pre-Golgi retention and degradation of the chimeric protein: the small amounts of molecules w hich exit from the ER are mostly covalent dimers. By contrast, when re tained by the KDEL sequence, cathepsin D is stable in the ER, indicati ng that retention is not sufficient to cause degradation. Replacing th e C-terminal cysteine with serine restores transport through the Golgi . As all chimeric cathepsin D constructs display comparable protease a ctivity in vitro, their different fates are not determined by gross al terations in folding. Thus, also out of its normal context, the mu cha in Cys575 plays a crucial role in quality control, mediating assembly, retention and degradation.