Mannose trimming targets mutant alpha(2)-plasmin inhibitor for degradation by the proteasome

We have previously characterized the molecular and cellular mechanisms of alpha(2)-plasmin inhibitor (alpha(2)PI) deficiency. The mutant alpha(2)PI-Nara and alpha(2)PI-Okinawa proteins were found to be retained and degraded in cells stably expressing these mutant forms of alpha(2)PI. Degradation of the two mutant alpha(2)PI proteins, mediated by proteasomes, occurred after a lag time of 1.5 h during which glucose trimming took place. The mutant alpha(2)PI proteins were not ubiquitinated. Inhibition of mannosidase activity blocked the degradation of the mutant alpha(2)PI proteins without resulting in any changes in their binding to calnexin. Inhibition of glucose removal completely blocked the interaction between the alpha(2)PI proteins and the molecular chaperone calnexin. Under these conditions, mannose residues were removed from the oligosaccharides even when glucose residues were not processed. With mannose removal, the glucose-untrimmed mutant forms of alpha(2)PI, which failed to bind to calnexin, were degraded by proteasomes. The initiation of mannose trimming was a prerequisite for their degradation. Our findings show that modification of oligosaccharides of the mutant forms of alpha(2)PI determines their recognition by the degradation apparatus and that mannose trimming is important for targeting the mutant alpha(2)PI proteins for the degradation pathway.