title: The human alpha-type proteasomal subunit HsC8 forms a double ringlike structure, but does not assemble into proteasome-like particles with the beta-type subunits HsDelta or HsBPROS26.
abstract: The eukaryotic proteasome is a barrel-shaped protease complex made up of four seven-membered rings of which the outer and inner rings may contain up to seven different alpha- and beta-type subunits, respectively. The assembly of the eukaryotic proteasome is not well understood. We cloned the cDNA for HsC8, which is one of the seven known human alpha-type subunits, and produced the protein in Escherichia coli. Recombinant HsC8 protein forms a complex of about 540 kDa consisting of double ringlike structures, each ring containing seven subunits. Such a structure has not earlier been reported for any eukaryotic proteasome subunit, but is similar to the complex formed by the recombinant alpha-subunit of the archaebacterium Thermoplasma acidophilum (Zwickl, P., Kleinz, J., and Baumeister, W. (1994) Nat. Struct. Biol. 1, 765-770). The ability of HsC8 to form alpha-rings suggests that these complexes may play an important role in the initiation of proteasome assembly in eukaryotes. To test this, we used two human beta-type subunits, HsBPROS26 and HsDelta. Both these beta-type subunits, either in the proprotein or in the mature form, exist in monomers up to tetramers. In contrast to the alpha- and beta-subunit of T. acidophilum, coexpression of the human beta-type subunits with HsC8 does not result in the formation of proteasome-like particles, which would be in agreement with the notion that proteasome assembly in eukaryotes is much more complex than in archaebacteria.
authors.affiliation: Department of Biochemistry, University of Nijmegen, 6500 HB Nijmegen, The Netherlands.
authors.forename: W L
authors.initials: WL
authors.lastname: Gerards
authors.forename: J
authors.initials: J
authors.lastname: Enzlin
authors.forename: M
authors.initials: M
authors.lastname: Häner
authors.forename: I L
authors.initials: IL
authors.lastname: Hendriks
authors.forename: U
authors.initials: U
authors.lastname: Aebi
authors.forename: H
authors.initials: H
authors.lastname: Bloemendal
authors.forename: W
authors.initials: W
authors.lastname: Boelens
mesh_terms: D003546:Cysteine Endopeptidases / Q000737:chemistry* / Q000648:ultrastructure; D006367:HeLa Cells; D006801:Humans; D008854:Microscopy, Electron; D008970:Molecular Weight; D009097:Multienzyme Complexes / Q000737:chemistry* / Q000648:ultrastructure; D046988:Proteasome Endopeptidase Complex; D011487:Protein Conformation; D011994:Recombinant Proteins / Q000737:chemistry; D013822:Thermoplasma
chemical_list: D009097:Multienzyme Complexes; D011994:Recombinant Proteins; D003546:Cysteine Endopeptidases; D046988:Proteasome Endopeptidase Complex