PURIFICATION AND PROPERTIES OF THE 26S PROTEASOME FROM THE RAT-BRAIN - EVIDENCE FOR ITS DEGRADATION OF MYELIN BASIC-PROTEIN IN A UBIQUITIN-DEPENDENT MANNER

A ubiquitin(Ub)/ATP-dependent proteolytic complex (26S proteasome) was highly purified from the rat brain. The brain 26S proteasome consiste d of 22-110 kDa subunits characteristic of the typical 26S proteasome on the basis of SDS-PAGE. The two-dimensional PAGE (NEPHGE and SDS-PAG E) pattern revealed that the pi values and molecular masses of the bra in 26S proteasome subunits were similar to those of the subunits of 26 S proteasomes purified from the rat liver and skeletal muscles. The en zymatic properties of the brain 26S proteasome were similar to those o f the liver complex and also to the Ub-conjugate degrading activity in the cerebral cortex extract. Furthermore, it was found that the brain 26S proteasome was capable of degrading the myelin basic protein in a Ub-dependent manner. These results indicate that the brain contains t he Ub-conjugate-degrading 26S proteasome, the subunit composition of w hich appears similar to those of the other tissues, and that the myeli n basic protein may be a candidate physiological substrate for the bra in 26S proteasome.