THE HUMAN ALPHA-TYPE PROTEASOMAL SUBUNIT HSC8 FORMS A DOUBLE RING-LIKE STRUCTURE, BUT DOES NOT ASSEMBLE INTO PROTEASOME-LIKE PARTICLES WITHTHE BETA-TYPE SUBUNITS HSDELTA OR HSBPROS26

The eukaryotic proteasome is a barrel-shaped protease complex made up of four seven-membered rings of which the outer and inner rings may co ntain up to seven different alpha- and beta-type subunits, respectivel y. The assembly of the eukaryotic proteasome is not well understood. W e cloned the cDNA for HsC8, which is one of the seven known human alph a-type subunits, and produced the protein in Escherichia coli. Recombi nant HsC8 protein forms a complex of about 540 kDa consisting of doubl e ringlike structures, each ring containing seven subunits. Such a str ucture has not earlier been reported for any eukaryotic proteasome sub unit, but is similar to the complex formed by the recombinant alpha-su bunit of the archaebacterium Thermoplasma acidophilum (Zwickl, P., Kle inz, 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 eu karyotes. To test this, we used two human beta-type subunits, HsBPROS2 6 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 o f proteasome-like particles, which would be in agreement with the noti on that proteasome assembly in eukaryotes is much more complex than in archaebacteria.