MOLECULAR-CLONING AND EXPRESSION OF A MULTIUBIQUITIN CHAIN BINDING SUBUNIT OF THE HUMAN 26S PROTEASE

S5a is a subunit of the 26S protease that binds and presumably selects multiubiquitinated proteins for destruction, We recently identified a n Arabidopsis protein, MBP1, that is physically, inmunologically and b iochemically similar to S5a from the human erythrocyte 26S protease. B ased upon the MBP1 cDNA sequence we have now isolated a HeLa cell cDNA coding for human S5a. The HeLa cDNA sequence is highly similar to MBP 1 and it encodes peptides obtained directly from human erythrocyte S5a . Moreover, expression of the isolated cDNA in E. coli results in a re combinant protein with an apparent molecular mass and multiubiquitin b inding properties that match those of human S5a obtained from the puri fied 26S enzyme.