Analysis of Structure and Function of Proteasome Derived from Rat Liver Microsome

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This investigation was aimed to characterize structural and enzymic properties of the ERb proteasome which we have isolated, for the first time in the world, from rat liver microsome fraction as an ER membrane-bound proteasome and to determine the structural element specific in the ERb proteasome, and we clarified the followings :1. On reverse-phase HPLC, in addition to β2 and α5 of 20S proteasome, ERb was found to contain extra subunits corresponding to β2 and α5 in 20S proteasome, which we referred as to B and L.2. TWO dimension gel electrophoresis revealed that B subunit is more acidic than β2 due to phosphorylation, and L subunit was essentially the same as α5, although slight difference in mobility was observed.3. On MALDI-TOF/MS, lysylendopeptidase digest of B showed the same pattern as that of β2.4. N-terminal amino acid analysis revealed that α5 has Thr and that of L was blocked. Taken together with the results of MALDI-TOF/MS, The N-terminal sequence of L was suggested as Ac-Met-Phe-Leu-Thr-where Thr was the same as N-terminal of α5.In summary, ERb contains one each of two distinctive subunits (referred to as B and L) which is absent from 20S. B is a counterpart of β2 in 20S and distinctive from β 2 in polarity and isoelectric point L is a counterpart of a5, having 3 additional residues to N-terminal of α5 and the N-terminal residue Met was acetylated. These structural feature may explain the property of ERb to bind to ER membrane.