PURIFICATION AND CHARACTERIZATION OF AN ENDOGENOUS INHIBITOR SPECIFICTO THE Z-LEU-LEU-LEU-MCA DEGRADING ACTIVITY IN PROTEASOME AND ITS IDENTIFICATION AS HEAT-SHOCK PROTEIN-90

We previously identified a xycarbonyl(Z)-Leu-Leu-Leu-4-methylcoumaryl- 7-amide (ZLLL-MCA) degrading activity in proteasome as a candidate for the regulator of neurite outgrowth. As its counterpart, we purified a protein from bovine brain that specifically inhibits the ZLLL-MCA deg rading activity in proteasome. This protein is heat stable and has no effect on the other catalytic activities in proteasome, or on the acti vities of trypsin, chymotrypsin, or m- and mu-calpains either. The mol ar ratio of inhibitor-to-proteasome that inhibits 50% of the ZLLL-MCA degrading activity of proteasome is 1:1. The inhibitory mechanism of t he inhibitor against proteasome is non-competitive. Finally, the inhib itor was identified as heat-shock protein 90 (HSP90) by partial amino acid sequencing and immunodetection. The results suggest that HSP90 in itiates neurite outgrowth through the inhibition of the ZLLL-MCA degra ding activity in proteasome.