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 an Arabidopsis protein, MBP1, that is physically, immunologically and biochemically similar to S5a from the human erythrocyte 26S protease. Based 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 MBP1 and it encodes peptides obtained directly from human erythrocyte S5a. Moreover, expression of the isolated cDNA in E. coli results in a recombinant protein with an apparent molecular mass and multiubiquitin binding properties that match those of human S5a obtained from the purified 26S enzyme.