IN-VIVO CHARACTERIZATION OF THE PROTEASOME REGULATOR PA28

A proteasome regulator, termed PA28, has been shown to modulate peptid ase activities of the proteasomes in vitro. Two different but homologo us PA28 molecules, designated as PA28 alpha and PA28 beta, have been c loned. Both alpha and beta polypeptides of PA28 are found in PA28 comp lexes isolated from cells, indicating that both are constituents of fu nctional PA28 complexes. Using antisera specific to PA28 alpha, PA28 b eta, and epitope-tagged PA28 molecules, we show that expression of PA2 8 alpha and PA28 beta is coordinately induced by various cytokines in different cell lines and that PA28 subunits and proteasomes have almos t identical half-lives. In addition, we show that PA28 complexes are a ssociated with 20 S but not 26 S proteasomes in vivo. Moreover, we dem onstrate that PA28 complex is a heterohexamer composed of both alpha a nd beta subunits with a stoichiometry of alpha(3) beta(3) in an altern ating order.